The safety and clinical impact of isoprinosine in HIV-infected individuals were assessed in a multicentre, randomized, double-blind, 24-week study phase, followed by an optional 24-week open treatment phase. The results of the double-blind phase have been reported. Of 866 HIV-seropositive patients randomized, 832 subjects were eligible for efficacy analysis. On completion of the double-blind phase, 596 patients started open treatment. All patients were evaluated with regard to progression to AIDS and/or death. Within 48 weeks, 10/412 (2.4%) patients assigned isoprinosine and 27/420 (6.4%) patients assigned placebo progressed to AIDS (P = 0.005). Intention-to-treat analysis showed identical results. Viewing the open treatment phase in isolation revealed no difference in progression rates between those treated and those not receiving the drug, perhaps reflecting the higher proportion of patients receiving zidovudine or PCP prophylaxis in the latter group. No severe adverse reactions or toxicities were observed. We conclude that HIV-seropositive patients without AIDS may be safely and effectively treated with isoprinosine.