From a retrospective study at the University of California, San Francisco, Medical Center, it is evident that pneumocystic carinii pneumonia is being seen more frequently as a secondary complication to the use of immunosuppressive drugs. This disease presents with nonspecific respiratory symptoms, therefore a high degree of suspicion and knowledge of the population at risk are necessary for an early diagnosis. Except for x-ray films of the chest, physical and laboratory studies are of minimal diagnostic value. In a patient with compromised immune defenses and respiratory distress, bilateral diffuse reticular infiltrates seen on a film of the chest are highly suggestive of pneumocystis carinii pneumonia. The diagnosis should be confirmed histologically because a variety of pathogens can cause these findings and each requires a specific treatment. At our institution, open thoracotomy is the method of choice for obtaining a lung biopsy specimen. Pentamidine isothionate is moderately effective against this usually fatal disease, but its effectiveness depends on beginning treatment early in the illness.