Transbronchial lung biopsies taken during episodes of acute lung rejection in heart-lung transplant patients were examined histologically. When the diagnosis was confirmed by microscopy, the patients were treated with augmented immunosuppression by high-dose corticosteroids. A second biopsy was obtained an average of 23.5 days after commencement of treatment. These biopsies were examined to determine the histological changes caused by treatment. In most cases, there were both quantitative and morphological differences between the infiltrates in acute rejection and in the remaining perivascular infiltrates after treatment. In acute rejection, 26 of 28 biopsies contained perivascular lymphocytic infiltrates, lymphocytes being large and blast-like. Although 20 of 28 follow-up biopsies still contained perivascular infiltrates, they were smaller and the lymphocytes smaller in size. Half the biopsies in rejection contained neutrophils, but less than half contained eosinophils in the perivascular infiltrates. After treatment, all these cells were less numerous. Another feature of treated rejection was the presence of haemosiderin around vessels suggesting earlier acute vascular injury. However, haemosiderin persists long after the cellular infiltrate has disappeared and cannot be considered a reliable feature of recently treated acute lung rejection. The bronchiolar infiltrates showed a similar pattern of responses to the perivascular infiltrates.