The effectiveness and antiretroviral activities of interferon-α in AIDS-related Kaposi's sarcoma was assessed in a non-randomised, phase-II clinical trial. 28 patients were treated with high-dose (27-36 MU) human recombinant interferon-α2a subcutaneously every day for 8 weeks. In patients with stable disease or showing a response, treatment was continued three times weekly until a complete response was achieved or there was progression. 12 of the 26 evaluable patients achieved a major response; 5 of these showed histologically confirmed complete responses. There was a significant increase in OKT4-positive cells in the responders and a significant decrease in HIV antigen (HIV-Ag) in the 7 responders with initially detectable HIV-Ag. Interferon-α is thus an effective treatment. The increase in OKT4-positive cells and the decrease in HIV-Ag seem to be significantly related to patients with tumour responses.