The safety and efficacy 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 separately. Of 866 HIV-seropositive individuals randomized, 832 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. Within 48 weeks, 10/412 patients (2.4%) assigned isoprinosine and 27/420 (6.4%) assigned placebo progressed to AIDS (p = 0.005; odds ratio: 2.8, 95% CI: 1.3-6.2). Intention-to-treat analysis showed identical results. No severe adverse reactions or toxicities were observed. We conclude that HIV-infected individuals without AIDS may be safely and effectively treated with isoprinosine.