Pneumocystis carinii was first described in 1909 by Chagas, who studied it in guinea pigs, and thought that it was a stage in the life cycle of Trypanosoma cruzi. Carini recognized a year later the same parasite in rats, but it remained for Delanoe and Delanoe to conclude that the organism was not a form of Trypanosoma cruzi. The latter investigators named it Pneumocystis carinii. It is now considered a protozoan and included in the class of Sporozoa. In this study of 36 patients with Pneumocystis carinii pneumonia a spectrum of histopathologic changes is demonstrated which, up to now, have not been considered to be classical features of the infection. These atypical microscopic findings were present in 69% of the cases as defined in this retrospective examination. As expected, the suggestion of the correct diagnosis of Pneumocystis carinii pneumonia on the basis of a frozen section and without the aid of appropriate special stains was extremely difficult in the majority of biopsies. It is important for the pathologist to maintain the possibility of this infection in his differential diagnosis when he examines a pulmonary biopsy from a patient who is immunologically compromised.