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Ayurvedic medicines prepared in India consist of Withania somnifera roots as one of the main ingredients. It is consumed as a dietary supplement around the world. The leaves of W. somnifera were used in the treatment of tumors and inflammation in several Asian countries. We have isolated twelve withanolides such as withaferin A (1), sitoindoside IX (2), 4-(1-hydroxy-2, 2-dimethylcyclpropanone)-2, 3-dihydrowithaferin A (3), 2, 3-dihydrowithaferin A (4), 24, 25-dihydro-27-desoxywithaferin A (5), physagulin D (1→6)-β-D-glucopyranosyl- (1→4)-β-D-glucopyranoside (6), 27-O-β-D-glucopyranosylphysagulin D (7), physagulin D (8), withanoside IV (9), and 27-O-β-D-glucopyranosylviscosalactone B (10), 4, 16-dihydroxy-5β, 6β-epoxyphysagulin D (11), viscosalactone B (12) from the leaves of this species. Compounds 1–12 and diacetylwithaferin A (13) were tested for their antiproliferative activity on NCI-H460 (Lung), HCT-116 (Colon), SF-268 (Central Nervous System; CNS and MCF-7 (Breast) human tumor cell lines. The inhibitory concentration to afford 50% cell viability (IC50) for these compounds was determined by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. Withaferin A and its derivatives exhibited inhibitory concentrations (50%) ranging from 0.24±0.01 to 11.6±1.9 μg/mL. Viscosalactone B (12) showed the 50% inhibition at concentrations ranging from 0.32±0.05 to 0.47±0.15 μg/mL whereas its 27-O-glucoside derivative (10) exhibited IC50 between 7.9±2.9 and 17.3±3.9 μg/ml. However, Physagulin D type withanolides showed either weak or no activity at 30 μg/mL. Therefore, incorporation of withanolides in the diet may prevent or decrease the growth of tumors in human. [Copyright &y& Elsevier]