To examine the role of sex steroid hormones in the development of autoimmune diseases, we studied five patients with Klinefelter's syndrome associated with autoimmune disease, three of whom had Sjögren's syndrome (SS) and two of whom had systemic lupus erythematosus (SLE). Serum testosterone (T) and LH levels, antinuclear antibodies (ANA) and rheumatoid factor (RF) titers, erythrocyte sedimentation rate (ESR), hemolytic complement (CH50) levels, and peripheral T lymphocyte subsets (OKT3+, OKT4+, and OKT8+) were measured before treatment, after 60 days of placebo treatment, and after 60 days of oral T undecanoate (TU) treatment. Before treatment and after placebo, with respect to normal men, the patients had lower serum T and higher LH levels, lower percentages and absolute values of OKT3+ (total T lymphocytes) and OKT8+ (suppressor/cytotoxic T lymphocytes) cells, and, consequently, an increased OKT4/OKT8 ratio. Hemolytic complement (CH50) in serum was below normal in the two patients with SLE, while it was normal in the patients with SS. The ESR was above normal in all patients, and all had high titers of ANA and RF. After TU therapy, serum T levels increased and LH levels decreased, but not to normal. OKT3+ and OKT8+ cells and the OKT4/OKT8 ratio became normal, and RF and ANA titers decreased. The CH50 level did not change in the SS patients, while it increased to normal in the two patients with SLE. The ESR decreased in all patients during therapy. Furthermore, after TU therapy, both the SS and SLE patients had a clinical remission of their autoimmune disease. Our results indicate a therapeutic effect of T on autoimmune diseases in patients with hypogonadism and Klinefelter's syndrome.