The leaves of Chromolaena odorata (Asteraceae) are exploited extensively in West and Central African ethnopharmacy for the treatment of a wide range of conditions, despite this being a non‐native species established in the last 50 years. With the objective of seeking bioactive principles, the nonvolatile compounds, an ethanolic (80% v/v) extract was made and fractionated. From the hexane‐soluble fraction, three compounds were isolated. Two of these, 5‐hydroxy‐7,4′‐dimethoxyflavanone and 2′‐hydroxy‐4,4′,5′,6′‐tetramethoxychalcone, have previously been identified in C. odorata leaves. The third was fully characterised spectroscopically and found to be 1,6‐dimethyl‐4‐(1‐methylethyl)naphthalene (cadalene), not previously isolated from the Asteraceae. All three compounds were tested for their cytotoxicity and anticancer properties. 2′‐Hydroxy‐4,4′,5′,6′‐tetramethoxychalcone was found to be both cytotoxic and anticlonogenic at 20 µm in cell lines Cal51, MCF7 and MDAMB‐468, and to act synergistically with the Bcl2 inhibitor ABT737 to enhance apoptosis in Cal51 breast cancer cells. Copyright © 2012 John Wiley & Sons, Ltd.