To determine the factors involved in the pathogenesis of thrombocytopenia frequently found in human immuno-deficiency virus (HIV) infection, we studied the clinical and laboratory findings of 35 Japanese HIV-infected hemophiliacs regarding their association with thrombocytopenia. Seventeen HIV-positive patients were thrombocytopenic. They had fewer CD4+ cells and were in more advanced stages of the disease, compared with the 18 patients without thrombocytopenia. We carried out the stepwise regression analysis on 32 patients in the early stage of HIV infection, with the platelet count as the dependent variable, and with the CD8+ cell count, serum cholinesterase, alanine aminotransferase (ALT), CD4+ cell count and white blood cell count as explanatory variables. The CD8+ cell count, serum cholinesterase, and ALT were entered into the regression model as explanatory variables of the platelet count with statistical significance. A positive linear correlation in these 32 patients between the CD8+ cell count and platelet count (r = 0.50, P < 0.01) was noted. We conclude that the decrease of the CD8+ cell count may play a role in the pathogenesis of thrombocytopenia in Japanese hemophiliacs in the early stage of HIV-infection.