A patient with Kaposi's sarcoma and the acquired immune deficiency syndrome became acutely febrile and dyspneic. Although chest roentgenograms and findings from arterial blood oxygenation studies were normal, bronchoscopy disclosed heavy Pneumocystis carinii infection. The patient was treated with trimethoprim-sulfamethoxazole with a rapid clinical response. It has been distinctly unusual to diagnose Pneumocystis without roentgenographic or blood gas abnormalities. Pneumocystis infection probably occurs as a wide spectrum of disease ranging from subclinical infection to frank pneumonitis. In the appropriate clinical setting, clinically significant Pneumocystis infection may be diagnosed despite the absence of an infiltrate or hypoxemia and early treatment may be beneficial.