Gonadal and sexual function was evaluated in 81 male patients with hairy cell leukemia (HCL). Forty-eight patients were receiving recombinant alpha 2b-interferon (INF) therapy, and 33 patients were receiving no systemic therapy at the time of evaluation. Among 28 patients with stable HCL who were receiving no systemic therapy, elevated serum follicle-stimulating hormone (FSH) was observed in two patients, while serum luteinizing hormone (LH) was normal in all. Serum testosterone levels were below normal in seven of 28 patients on no systemic therapy. Hormone levels were followed prospectively for a median of 8 months in 11 patients receiving INF without prior chlorambucil therapy. FSH became elevated in only one of eight patients with normal pretreatment values, LH remained normal in all patients, and testosterone transiently fell in two of four patients with normal pretreatment values. In addition, 20 of 29 patients receiving INF who had normal FSH and LH levels when first examined continued to have normal levels during a median of 10 months of INF therapy. Patients receiving INF generally reported either no change or improvement in sexual function as therapy progressed. We conclude that most patients with stable HCL have normal testicular function and that INF therapy does not appear to produce significant gonadal toxicity.