Ten non-AIDS patients with Pneumocystis carinii pneumonia were studied. While the 2 patients with adult T cell leukemia had longer prodromes, the other 8 patients had acute onset. At presentation a chest radiograph revealed an abnormal bilateral diffuse shadow in all cases. In 8 patients, diagnostic material was obtained by transbronchiallung biopsy and/or bronchoalveolar lavage, and in 2 patients at postmortem. At the time of diagnosis the serum lactate dehydrogenase value was much higher than prior to the acute illness, and the AaD0 2 gradient was highly increased: These appear to be useful as markers for an initial diagnosis. Other opportunistic organisms were isolated in 5 patients. The concomitant use of pentamidine and cotrimoxazole was relatively well tolerated, but with a high incidence of treatment failure. Corticosteroids appeared to be effective as an adjunctive therapy.