diff --git a/src/ast/nodes/shared/callHasEffects.js b/src/ast/nodes/shared/callHasEffects.js
index ce48fc0..2c37442 100644
--- a/src/ast/nodes/shared/callHasEffects.js
+++ b/src/ast/nodes/shared/callHasEffects.js
@@ -70,6 +70,11 @@ export default function callHasEffects ( scope, callee, isNew ) {
 			if ( fnHasEffects( node, isNew && isES5Function( node ) ) ) return true;
 		}
 
+		else if ( /Class/.test( node.type ) ) {
+			// TODO find constructor (may belong to a superclass)
+			return true;
+		}
+
 		else if ( isReference( node ) ) {
 			const flattened = flatten( node );
 			const declaration = scope.findDeclaration( flattened.name );
diff --git a/test/form/class-constructor-side-effect/_config.js b/test/form/class-constructor-side-effect/_config.js
new file mode 100644
index 0000000..5c8a4a0
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_config.js
@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'preserves side-effects in a class constructor (#1233)'
+};
diff --git a/test/form/class-constructor-side-effect/_expected/amd.js b/test/form/class-constructor-side-effect/_expected/amd.js
new file mode 100644
index 0000000..1b939aa
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_expected/amd.js
@@ -0,0 +1,11 @@
+define(function () { 'use strict';
+
+	class Foo {
+		constructor () {
+			console.log( 'Foo' );
+		}
+	}
+
+	new Foo;
+
+});
diff --git a/test/form/class-constructor-side-effect/_expected/cjs.js b/test/form/class-constructor-side-effect/_expected/cjs.js
new file mode 100644
index 0000000..ad3db87
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_expected/cjs.js
@@ -0,0 +1,9 @@
+'use strict';
+
+class Foo {
+	constructor () {
+		console.log( 'Foo' );
+	}
+}
+
+new Foo;
diff --git a/test/form/class-constructor-side-effect/_expected/es.js b/test/form/class-constructor-side-effect/_expected/es.js
new file mode 100644
index 0000000..705e350
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_expected/es.js
@@ -0,0 +1,7 @@
+class Foo {
+	constructor () {
+		console.log( 'Foo' );
+	}
+}
+
+new Foo;
diff --git a/test/form/class-constructor-side-effect/_expected/iife.js b/test/form/class-constructor-side-effect/_expected/iife.js
new file mode 100644
index 0000000..a047aa4
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_expected/iife.js
@@ -0,0 +1,12 @@
+(function () {
+	'use strict';
+
+	class Foo {
+		constructor () {
+			console.log( 'Foo' );
+		}
+	}
+
+	new Foo;
+
+}());
diff --git a/test/form/class-constructor-side-effect/_expected/umd.js b/test/form/class-constructor-side-effect/_expected/umd.js
new file mode 100644
index 0000000..c4409fe
--- /dev/null
+++ b/test/form/class-constructor-side-effect/_expected/umd.js
@@ -0,0 +1,15 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, (function () { 'use strict';
+
+	class Foo {
+		constructor () {
+			console.log( 'Foo' );
+		}
+	}
+
+	new Foo;
+
+})));
diff --git a/test/form/class-constructor-side-effect/main.js b/test/form/class-constructor-side-effect/main.js
new file mode 100644
index 0000000..705e350
--- /dev/null
+++ b/test/form/class-constructor-side-effect/main.js
@@ -0,0 +1,7 @@
+class Foo {
+	constructor () {
+		console.log( 'Foo' );
+	}
+}
+
+new Foo;