bcl-2 is a proto-oncogene with a regulatory role in many conditions due to its marked inhibitory action on apoptosis. Reports regarding the effect of hemodialysis (HD) on apoptosis of mononuclear cells and in association with bcl-2 expression in particular, are controversial. The aim of the present study was to examine in vivo the influence of an HD session on bcl-2 expression of lymphocytes and monocytes. We measured quantitative bcl-2 expression with flow cytometry, in terms of antibodies bound per cell, in blood samples taken from 44 HD patients before and after an HD session. 27 patients (group I) were dialyzed with synthetic-type membranes and 17 (group II) with cellulose-type membranes. bcl-2 expression increased statistically significantly in lymphocytes (1,616 ± 718 to 1,894 ± 715 molecules/cell, p < 0.01) at the end of HD. Monocyte expression of bcl-2 was lower than in lymphocytes and almost did not change after the HD session (654 ± 446 to 698 ± 375 molecules/cell, p = NS). Comparison between the two groups did not reveal a significant difference in either the baseline bcl-2 expression or in the value of the increase after HD. We conclude that HD seems to decrease lymphocyte apoptosis independent of the biocompatibility of the dialyzer membrane.