Serial pulmonary function studies were performed during the convalescent stage on a patient who survived biopsy-proved Pneumocystis carinii pneumonia. A lung biopsy specimen was obtained 5 months after the onset of the acute illness. These studies documented the presence of diffuse interstitial fibrosis of the lung and strongly suggested that the fibrosis developed as a sequel to Pneumocystis carinii infection. This observation is consistent with the theory that diffuse interstitial pneumonitis of infectious cause may progress to diffuse interstitial fibrosis.