We tested the effects of hydroxychloroquine (HCQ), an anti-rheumatic drug, on the viability of chronic lymphocytic leukemia (CLL) cells. HCQ induced a decrease in cell viability in a dose- and time-dependent manner. The mean LC50 calculated for the cells of 20 patients was 32 +/- 7 microg/ml (range, 10-75 microg/ml). We observed a large increase in apoptotic cell number after 24 h of incubation with 50 microg/ml HCQ (55 +/- 6 vs. 23 +/- 3% in medium alone, p < 0.001). Indeed, HCQ in leukemic cells induced the features of apoptosis (cell shrinkage, decrease in mitochondrial transmembrane potential, phosphatidylserine externalization, chromatin condensation and DNA fragmentation). HCQ had marked selective cytotoxicity when compared with normal blood mononuclear cells, in which the LC50 was >100 microg/ml at 24 h. HCQ induced the proteolytic cleavage of poly(ADP(adenosine 5'-diphosphate)ribose) polymerase (PARP) and increased the activity of caspase-3. The expression of bcl-2 and bax proteins was significantly modified after incubation with the drug and HCQ activity against CLL cells occurred independently of the presence of IL-4, sCD40L and bone marrow stromal cells.