To investigate the influence of apheresis on lymphocyte subsets, we compared lymphocyte subsets of repeated apheresis donors (RADs) with those of nondonor controls (NDCs).Males were divided into three age groups; group A (age range 19-34 years, RADs; n=19,30+ 11 donations/2y, NDCs; n=16), group B (35-54 years, RADs; n=16, 32+10 donations/2y, NDCs; n=20) and group C (55-67 years, RADs; n14, 32+8 donations/2y, NDCs; n=8). Females were divided into two age groups; group D (19-34 years, RADs; n=14, 23+8 donations/2y, NDCs; n= 18) and group E (35-62 years, RADs; n=12, 22+11donations/2y, NDCs; n=7). In lymphocyte subsets, CD3+T cells, CD4+cells, CD8+ cells, CD19+B cells and CD56+CD3-natural killer (NK) cells were analyzed by flow cytometry and the CD4/CD8 ratio was calculated. The total white blood cell count (TWBC) and absolute lymphocyte count (ALC) were determined using an automatic cell counter. The data of RADs were compared to those of NDCs in each group.In groups A, B and D, there were no significant differences in the percentages of lymphocyte subsets, or in ALCs or TWBCs. In group C, the percentage and absolute numbers of CD4+(RADs; 39.2+7.3%, NDCs; 47.3+4.1%, p<0.01, RADs; 684+185/u1, NDCs; 898+256/u1, p<0.05) and CD4/CD8 ratio (RADs; 1.65+0.74, NDCs; 2.28+0.37, p <0.05) were significantly decreased in RADs. In group D, the percentage and the absolute numbers of CD3+T cells (RADs; 68.5+5.9%, NDCs; 73.9+5.0%, p<0.02, RADs; 1, 162+259/ul, NDCs; 1,372+273/ul, p< 0.05) were significantly decreased in RADs. However, these findings indicate that repeated apheresis did not seriously affect lymphocyte subsets of RADs, since both the percentages and absolute numbers of lymphocyte subsets in RADs were within normal ranges.