Patients with AIDS but without pneumocystis carinii pneumonia, patients with advanced AIDS-related complex (ARC), and asymptomatic patients with less than 200 CD4-positive lymphocytes/mm3 were randomized to one of two groups: group I: Inhalation of 300 mg of pentamidine every 28 days; group II: Inhalation of placebo (300 mg of Na isethionate) every 28 days. From May to November 28, 1989, 160 patients have entered the trial. Inhalations were well tolerated, with only a 6% use of bronchodilators and a 15% incidence of cough. Until now five patients died, none of them drug related. So far, six patients have developed pneumocystis carinii pneumonia; four of these were on pentamidine, and two on placebo. Five of the six cases occurred before the second inhalation. Recruitment will continue until 250 patients are enrolled.