Although the effects of androgen deficiency in the immune system have long been appreciated, little is known about the immunological features of patients with Klinefelter's syndrome (KS). On the other hand, interest in androgens as a possible treatment for some autoimmune diseases is growing. In the present study, some immunological parameters were evaluated in 26 patients with KS prior to androgen replacement treatment (ART) and the results were compared with those in 19 healthy control subjects. Patients were then treated with testosterone for 6 months and the pre- and post-treatment findings were compared. Serum levels of IgG, IgA, IgM, C3c and C4 were measured by nephelometry and lymphocyte subsets and CD4+/CD8+ ratios were examined by flow cytometry. IL-2 and IL-4 levels were measured by ELISA. Pretreatment levels of the serum IgA, IgG, IgM, IL-2 and IL-4 of the patients were higher than those of the controls and were all decreased significantly following ART. The pretreatment absolute numbers and percentages of CD3+, CD4+, CD19+ cells and CD4+/CD8+ ratios of patients with KS were higher than those of the controls and were all decreased with ART. Percentages of CD8+ cells were increased significantly, while C3 and C4 levels were both significantly decreased after ART. It is concluded that the lack of testosterone in patients with KS enhances cellular and humoral immunity and that ART may suppress this.