A series of new imidazole carboxylic esters (carbamates) and N-acylimidazole derivatives of betulin and betulinic acid (14-29) have been synthesized. The new compounds were screened for in vitro cytotoxicity activity against human cancer cell lines HepG2, Jurkat and HeLa. A number of compounds have shown IC(50) values lower than 2 microM against the cancer cell lines tested and the vast majority has shown a better cytotoxicity profile than betulinic acid, including the betulin derivatives. N-Acylimidazole derivatives 26 and 27 (IC(50) 0.8 and 1.7 microM in HepG2 cells) and the C-3 carbamate derivative 16 (IC(50) 2.0 microM in HepG2 cells) were the most promising compounds. Based on the observed cytotoxicity, structure-activity relationships have been established.