Pneumocystosis, the most common opportunistic infection associated with the acquired immunodeficiency syndrome, is usually restricted to the lungs and results in severe bilateral pneumonia, which is fatal unless vigorously treated. Rare cases have been reported in which involvement of other organs or disseminated disease occurred in addition to the pulmonary lesions. Pentamidine, an efficient drug used intravenously for the treatment of pulmonary pneumocystosis, has also recently been used in aerosolized form for the prevention of Pneumocystis infection in patients with the acquired immunodeficiency syndrome. In the present case, widely disseminated, though symptomless, pneumocystosis developed in a human immunodeficiency virus-positive individual treated prophylactically with aerosolized pentamidine. Despite heavy multiorgan infection with Pneumocystis carinii, the lungs revealed no microorganisms or characteristic inflammatory lesions. This case indicates that aerosolized pentamidine, while efficient against the pulmonary infection, may not produce fungicidal blood levels sufficient for the prevention of disseminated pneumocystosis.