Pneumocystis carinii pneumonia (PCP) is the most common index diagnosis in patients with acquired immunodeficiency syndrome (AIDS). Eighty percent of AIDS patients will eventually develop PCP. Common presenting symptoms are shortness of breath, cough, weight loss, and fever. The chest radiograph typically reveals a diffuse, bilateral, symmetric, fine-to-medium reticulonodular pattern of opacities. Upper lobe disease has been reported recently in patients who have undergone prophylactic therapy with aerosolized pentamidine. Pneumatoceles, or thin-walled air-containing cysts, have been noted in 10% of patients with PCP. Lymphadenopathy and pleural effusion are extremely rare. Therapy with various agents is generally successful, resulting in clinical and radiographic improvement within 7 to 10 days. Recurrence of PCP is common.