Four ent-kaurene diterpenes were isolated from the leaves of Laetia thamnia L.: ent-kaur-16-en-19-oic acid (1a), ent-3beta-hydroxykaur-16-ene (2), ent-kaur-16-en-3alpha,19-diol (3a), and ent-17-hydroxykaur-15-en-19-oic acid (4). The methyl ester (1b) of compound 1a and the acetate diester (3b) of compound 3a were prepared, and all compounds were evaluated for cytotoxicity against human prostate (22Rv1, LNCaP), colon (HT29, HCT116, SW480, SW620), and breast (MCF-7) tumor cells at concentrations ranging from 6 to 50microg/mL. The kaurenes showed activity in all cell lines tested, with the prostate cells demonstrating the most sensitivity as follows: 22 Rv1 cells towards 1a (IC(50) 5.03microg/mL) and 1b (IC(50) 6.81microg/mL), and LNCaP towards 2 (IC(50) 12.83microg/mL) and 4 (IC(50) 17.63microg/mL).