Atypical Pneumocystis carinii pneumonia: the potential hazards of empiric treatment.

Pneumocystis carinii pneumonia occurred in two patients with unusual clinical findings. In one case, P carinii infection was diagnosed for the first time in a patient with Crohn's disease. Due to the lack of typical features in both cases an empiric trial of antimicrobial therapy could have delayed diagnosis and initiation of specific treatment for P carinii. Contrariwise, without histologic confirmation, specific therapy for P carinii pneumonia might have been mistakenly discontinued in the second patient. Empiric therapy with trimethoprim-sulfamethoxazole may cause rapid disappearance of P carinii cysts from pulmonary tissues and cause subsequent biopsies to be falsely negative in patients who fail to respond to treatment.