A new benzofuran derivative (1) was isolated from the roots of Petasites hybridus and its structure was determined as 1-(6-hydroxy-2-isopropenyl-1-benzofuran-5-yl)-1-ethanone by spectroscopic data and X-ray crystallographic analysis. The compound 1 showed moderate inhibitory activity on human breast cancer MCF-7 cells proliferation in vitro with an IC50 value of 48 mu mol/L. The interaction of bovine serum albumin (BSA) and DNA with compound 1 were studied in an aqueous solution under physiological conditions by UV-vis spectroscopy. (C) 2011 Phytochemical Society of Europe. Published by Elsevier B.V. All rights reserved.