diff --git a/lib/features/lasso-tool/LassoTool.js b/lib/features/lasso-tool/LassoTool.js
index 9776d8086..a106aff36 100644
--- a/lib/features/lasso-tool/LassoTool.js
+++ b/lib/features/lasso-tool/LassoTool.js
@@ -21,6 +21,8 @@ import {
  * @typedef {import('../mouse/Mouse').default} Mouse
  * @typedef {import('../selection/Selection').default} Selection
  * @typedef {import('../tool-manager/ToolManager').default} ToolManager
+ *
+ * @typedef {import('../../util/Types').Rect} Rect
  */
 
 var LASSO_TOOL_CURSOR = 'crosshair';
@@ -109,7 +111,9 @@ export default function LassoTool(
 
   // lasso interaction implementation
 
-  eventBus.on('lasso.end', function(event) {
+  eventBus.on('lasso.end', 0, function(event) {
+
+    var context = event.context;
 
     var bbox = toBBox(event);
 
@@ -117,7 +121,9 @@ export default function LassoTool(
       return element;
     });
 
-    self.select(elements, bbox);
+    var add = hasSecondaryModifier(event);
+
+    self.select(elements, bbox, add ? context.selection : []);
   });
 
   eventBus.on('lasso.start', function(event) {
@@ -126,6 +132,8 @@ export default function LassoTool(
 
     context.bbox = toBBox(event);
     visuals.create(context);
+
+    context.selection = selection.get();
   });
 
   eventBus.on('lasso.move', function(event) {
@@ -169,7 +177,12 @@ LassoTool.$inject = [
   'mouse'
 ];
 
-
+/**
+ * Activate lasso.
+ *
+ * @param {MouseEvent} event
+ * @param {boolean} [autoActivate=false]
+ */
 LassoTool.prototype.activateLasso = function(event, autoActivate) {
 
   this._dragging.init(event, 'lasso', {
@@ -181,6 +194,12 @@ LassoTool.prototype.activateLasso = function(event, autoActivate) {
   });
 };
 
+/**
+ * Activate selection.
+ *
+ * @param {MouseEvent} event
+ * @param {boolean} [autoActivate=false]
+ */
 LassoTool.prototype.activateSelection = function(event, autoActivate) {
 
   this._dragging.init(event, 'lasso.selection', {
@@ -189,16 +208,30 @@ LassoTool.prototype.activateSelection = function(event, autoActivate) {
     cursor: LASSO_TOOL_CURSOR,
     data: {
       context: {}
-    }
+    },
+    keepSelection: true
   });
 };
 
-LassoTool.prototype.select = function(elements, bbox) {
+/**
+ * Select elements within the given bounds.
+ *
+ * @param {Element[]} elements
+ * @param {Rect} bbox
+ * @param {Element[]} [previousSelection]
+ */
+LassoTool.prototype.select = function(elements, bbox, previousSelection = []) {
   var selectedElements = getEnclosedElements(elements, bbox);
 
-  this._selection.select(values(selectedElements));
+  this._selection.select([
+    ...previousSelection,
+    ...values(selectedElements)
+  ]);
 };
 
+/**
+ * Toggle the lasso tool.
+ */
 LassoTool.prototype.toggle = function() {
   if (this.isActive()) {
     return this._dragging.cancel();
@@ -209,6 +242,11 @@ LassoTool.prototype.toggle = function() {
   this.activateSelection(mouseEvent, !!mouseEvent);
 };
 
+/**
+ * Check if the lasso tool is active.
+ *
+ * @returns {boolean}
+ */
 LassoTool.prototype.isActive = function() {
   var context = this._dragging.context();
 
@@ -216,7 +254,6 @@ LassoTool.prototype.isActive = function() {
 };
 
 
-
 function toBBox(event) {
 
   var start = {
diff --git a/test/spec/features/lasso-tool/LassoToolSpec.js b/test/spec/features/lasso-tool/LassoToolSpec.js
index 2881523a0..184991d4e 100755
--- a/test/spec/features/lasso-tool/LassoToolSpec.js
+++ b/test/spec/features/lasso-tool/LassoToolSpec.js
@@ -19,7 +19,6 @@ import draggingModule from 'lib/features/dragging';
 
 describe('features/lasso-tool', function() {
 
-
   beforeEach(bootstrapDiagram({
     modules: [
       modelingModule,
@@ -102,7 +101,6 @@ describe('features/lasso-tool', function() {
         height: 220
       };
 
-
       // when
       lassoTool.select(elements, bbox);
 
@@ -113,36 +111,73 @@ describe('features/lasso-tool', function() {
       expect(selectedElements[0]).to.equal(childShape2);
       expect(selectedElements[1]).to.equal(childShape3);
     }));
+
+
+    it('shoud select elements in bbox and previously selected elements', inject(function(lassoTool, selection) {
+
+      // given
+      selection.select([ childShape ]);
+
+      var elements = [ childShape2, childShape3 ];
+
+      var bbox = {
+        x: 175,
+        y: 0,
+        width: 120,
+        height: 220
+      };
+
+      // when
+      lassoTool.select(elements, bbox, selection.get());
+
+      // then
+      var selectedElements = selection.get();
+
+      expect(selectedElements.length).to.equal(3);
+      expect(selectedElements[0]).to.equal(childShape);
+      expect(selectedElements[1]).to.equal(childShape2);
+      expect(selectedElements[2]).to.equal(childShape3);
+    }));
+
   });
 
 
-  describe('visuals', function() {
+  describe('#activateLasso', function() {
 
     beforeEach(inject(function(dragging) {
       dragging.setOptions({ manual: true });
     }));
 
 
-    it('should show lasso box', inject(function(lassoTool, canvas, dragging) {
+    it('should select after lasso', inject(function(lassoTool, dragging, selection, elementRegistry) {
 
       // when
       lassoTool.activateLasso(canvasEvent({ x: 100, y: 100 }));
       dragging.move(canvasEvent({ x: 200, y: 300 }));
+      dragging.end();
 
       // then
-      expect(canvas._svg.querySelector('.djs-lasso-overlay')).to.exist;
+      expect(selection.get()).to.eql([ elementRegistry.get('child') ]);
     }));
 
+  });
 
-    it('should select after lasso', inject(function(lassoTool, dragging, selection, elementRegistry) {
+
+  describe('visuals', function() {
+
+    beforeEach(inject(function(dragging) {
+      dragging.setOptions({ manual: true });
+    }));
+
+
+    it('should show lasso box', inject(function(lassoTool, canvas, dragging) {
 
       // when
       lassoTool.activateLasso(canvasEvent({ x: 100, y: 100 }));
       dragging.move(canvasEvent({ x: 200, y: 300 }));
-      dragging.end();
 
       // then
-      expect(selection.get()).to.eql([ elementRegistry.get('child') ]);
+      expect(canvas._svg.querySelector('.djs-lasso-overlay')).to.exist;
     }));
 
   });