Chloroform extract (CE) and fractions obtained from Aldama arenaria roots were evaluated for their in vitro antiproliferative activity against 10 human tumor cell lines [leukemia (K-562), breast (MCF-7), ovary expressing a multidrug-resistant phenotype (NCI/ADR-RES), melanoma (UACC-62), lung (NCI-H460), prostate (PC-3), colon (HT29), ovary (OVCAR-3), glioma (U251), and kidney (786-0)]. CE presented weak to moderate antiproliferative activity (mean log GI(50) 1.07), whereas fractions 3 and 4, enriched with pimarane-type diterpenes [ent-pimara-8(14),15-dien-19-oic acid and ent-8(14),15-pimaradien-3 beta-ol], presented moderate to potent activity for most cell lines, with mean log GI(50) of 0.62 and 0.59, respectively. The results showed promising in vitro antiproliferative action of the samples obtained from A. arenaria, with the best results for NCl/ADR-RES, HT29, and OVCAR-3, and TGI values ranging from 5.95 to 28.71 mu g.ml(-1), demonstrating that compounds of this class may be potential prototypes for the discovery of new therapeutic agents.