Background - A study was performed to identify the clinical, radiographic, and histopathological features of interstitial penumonitis in patients infected with the human immunodeficiency virus. Methods - A retrospective review was made ofthe case notes, chest radiographs, and histopathological results ofseven HIV- 1 antibody positive patients with symptomatic diffuse pulmonary disease and a pathological diagnosis ofnon-specific interstitial pneumonitis. Results - All patients had dyspnoea, with or without cough, and chest radiographs showing diffuse infiltrates. The arterial oxygen tension ranged widely from 5 9 to 13-1 kPa. The initial clinical diagnosis was Pneumocystis carinii pneumonia in most cases. The pathological diagnosis was made by transbronchial biopsy in one case and by open lung biopsy in six cases. The interstitial pneumonitis consisted of a patchy lymphocytic infiltrate composed of B cells in focal aggregates and T cells in a more diffuse distribution. The T cell population was a mixture of CD4+ and CD8 + cells. The histological findings contrast with the more extensive infiltrate of predominantly CD8 + lymphocytes seen in HIV-associated lymphocytic interstitial pneumonitis which occurs mainly in children. The condition ran a subacute course. Three patients spontaneously improved and three improved with steroid therapy. Long term survival was less than three years, the prognosis being determined by other infective or neoplastic complications. Conclusions - Non-specific interstitial pneumonitis usually presents with an illness resembling Pneumocystis carinii pneumonia but occurs when the CD4 and total lymphocyte counts are still preserved. The pneumonitis resolves spontaneously or responds to steroids, and does not itself lead direcdy to the patient's death. It does, however, appear to mark a downturn in the course of HIV infection.