Pulmonary microcalcifications were identified in 13 patients with Pneumocystis carinii pneumonia (PCP) and acquired immune deficiency syndrome (AIDS). Several patterns of calcification were noted including bubbly, plate-like, elongate, and conchoidal forms. All calcifications contained P. carinii cysts within them as demonstrated by Grocott's methenamine silver stain. In eight of the 13 cases, the typical intra-alveolar exudate of P. carinii was absent. Therefore, the calcifications were the only histopathologic indication of prior P. carinii infection. The above-mentioned calcifications are important because in five cases they occurred without a prior diagnosis of PCP, subsequent therapy, or prophylaxis. This indicates that in some patients there is destruction of P. carinii organisms with subsequent calcification, and this can occur without therapy.