Phytochemical study of Gochnatia polymorpha (Less) Cabr. ssp. floccosa Cabr. trunk bark, guided by antiproliferative assays on 10 human cancer cell lines and the VERO cell line, yielded six known compounds identified as the triterpene bauerenyl acetate, the guaianolide 11α,13-dihydrozaluzanin C and the dimeric guaianolides 10-desoxygochnatiolide A, gochnatiolide A, 8-hydroxi-10-desoxygochnatiolide A and 8-hydroxigochnatiolide A. Extracts, fractions of extracts and isolated compounds were tested in vitro against a panel of human cancer cell lines, including U251 (glioma, CNS), UACC-62 (melanoma), MCF-7 (breast), NCI-ADR/RES (drug-resistant ovarian), 786.0 (kidney), NCI-H460 (lung, no small cells), PC-3 (prostate), OVCAR-3 (ovarian), HT-29 (colon), K562 (leukemia) and against the VERO no cancer cell line. Bauerenyl acetate was inactive, while 11α,13-dihydrozaluzanin C showed weak activity against UACC62 and the VERO cell line. The most active compounds were 10-desoxygochnatiolide A and gochnatiolide A, which inhibited the growth of kidney, melanoma, ovarian-resistant and glioma cell lines with values of TGI (total growth inhibition) varying 0.21–1.09 µg/mL. This is the first report about cytotoxic activity of dimeric lactones against cell lines. Copyright © 2011 John Wiley & Sons, Ltd.