More than 25 000 serum specimens have been tested for antibody to human T-lymphotropic virus type III (HTLV-III) at the Laboratory Centre for Disease Control, Ottawa, since August 1984. In 1985 the prevalence rates of antibody positivity among selected risk groups were as follows: patients with Kaposi's sarcoma, 77%; patients with acquired immune deficiency syndrome (AIDS) or AIDS-related complex (ARC), 66%; patients with hemophilia, 65%; symptomatic homosexual men, 48%; cohabitants of patients with AIDS, ARC or antibody to HTLV-III, 24%; and intravenous drug abusers, 13%. No case of accidental parenteral exposure has resulted in seroconversion. Eight cases of AIDS, all in antibody-positive patients, have been associated with blood transfusions. A testing protocol based on risk-group information is proposed for diagnostic laboratories.