Coumarin is found in many medicinal plants and therefore also used in phytomedicine for the treatment of venous diseases. The metabolic pathways of coumarin in the human body lead to the intermediate 7-hydroxy-coumarin and consequent glucuronidation in the intestine and liver. The antitumor activities of coumarin (C) and its known metabolite 7-hydroxy-coumarin (7-OH-C) were tested in several human tumor cell lines. C as well as 7-OH-C inhibited cell proliferation of a gastric carcinoma cell line, a colon-carcinoma cell line (Caco-2), a hepatoma-derived cell line (HepG2) and a lymphoblastic cell line (CCRF CEM) in a concentration-dependent way, the IC50-values were 1.59-3.57 mM for C and 0.68-2.69 mM for 7-OH-C. The glucuronide of 7-OH-C was ineffective in this respect.