We evaluated the effects of 13-cis retinoic acid (13-cis RA) on the growth of peripheral blood mononuclear cells (PBMC) obtained from 12 patients with adult T cell leukemia (ATL). In general, 13-cis RA potently inhibited the growth of PBMC from ATL patients. However, the sensitivity of the cells to 13-cis RA-induced growth inhibition varied among the patients. The ATL patients fell into three groups (hypersensitive, sensitive and resistant to 13-cis RA) according to the percent reduction of 3H-thymidine incorporation before and after treatment with 13-cis RA. Agarose gel electrophoresis of total genomic DNA from a patient sensitive to 13-cis RA provided evidence of the DNA fragmentation indicative of apoptosis. The ability of 13-cis RA to induce apoptosis in PBMC from ATL patients suggests that retinoids may be useful in the treatment of ATL.