Sixteen patients with P. carinii infection were studied between 1977 and 1989; 12 were male and the age ranged from 4 months to 53 years old. 19 samples, 5 from autopsy, 7 from biopsy and 7 cytological aspirates were stained with hematoxylin-eosin, Gomori-Grocott, Giemsa and 0 toluidine. Autopsy material was studied by transmission electron microscopy. The diagnosis was made premortem in 11 patients. AIDS was the underlying disease in 6 patients, the rest having other immunodepressant conditions. P. carinii infection was limited to the lung in all patients. The Gomori Grocott stain gave positive results in all cases; cytological studies were generally noncontributory. Opportunistic agents were found in 4 patients (aspergillus, cytomegalovirus and C. neoformans).