Pneumocystis is typically described in the immunodepressed. We report a case of pneumocystis occurring in a patient without known depression of the immune system. The patient, aged 50, was hospitalised for a diffused infiltration pneumonia which developed sub-acutely, and presented with increasing dyspnoea of effort, thoracic pain and a disturbances of general health. The initial assessment did not reveal any risk factors for HIV infection nor any past history of note. The diagnosis of pneumocystis was confirmed by the presence of Pneumocystis carinii in the bronchoalveolar lavage from two samples. There was a favourable outcome following the prescription of Cotrimoxazole for three months and steroid therapy. HIV serology was negative and the sub-population of lymphocytes was normal. A search for neoplasia or systematic disease remained negative.