To assess the prevalence of atypical PCP patterns in AIDS patients, the chest films of 300 patients with clinical and laboratory diagnosis of PCP were reviewed. We considered as atypical patterns the finding of an asymmetric lesion, the involvement of apical regions, pleural effusion, hilar and/or mediastinal lymphadenopathy, parenchymal nodules and pneumatoceles. In some patients more than one of these patterns was found. Atypical patterns were observed in 32% of cases and consisted of: unilateral involvement in 11% of cases, apical involvement in 6%, pleural effusion in 9%, hilar and/or mediastinal lymph nodes in 4%, parenchymal nodules in 6% and finally pneumatocele in 12% of cases.