Four pentacyclic triterpenoids were obtained from the leaves of Combretum oliviforme Chao, 3beta-hydroxyolean-12-en-28-oic acid (1), 23-O-[alpha-L-(4'-acetylrhamnopyranosyl)]-imberbic acid (2), 23-acetoxy-3beta-acetylimberbic acid-29-methyl ester (3), and 23-O-[alpha-L-rhamnopyranosyl]-1,3beta-diacetylimberbic acid (4). Hydrolysis of 2 and 4 gave 23-hydroxyimberbic acid (5). The structures were elucidated by NMR, electrospray ionization mass spectrometry (ESIMS) and comparison with literature data. Compounds 1, 2, 3 and 4 were isolated from C. oliviforme Chao leaves for the first time and 3 for the first time from any natural source. All compounds were tested in vitro for their activity against human lung cancer cell line SPC-A-1, human erythroleukaemic line K562 and human gastric cancer SGC-7901 cells. Compounds 1, 3, 4 and 5 had cytotoxic activity for the three cell lines with IC50 0.69-69.68 microM. These results suggest that the presence of acetyl group in the triterpene aglycone structure plays an essential role for cytotoxic activity.