Patients with confirmed interstitial pneumonia were initially classified histologically into "desquamative" (n = 40) and "usual" (n = 53) types, and followed for one to 22 years. Both the diagnosis and the extent of fibrosis affected the course and response to therapy. Mortality in desquamative interstitial pneumonia was 27.5 per cent, and mean survival 12.2 years, as compared with 66.0 per cent and 5.6 years in usual interstitial pneumonia (P less than 0.01). Without treatment, 21.9 per cent with the desquamative but none with the usual type improved. With corticosteroid therapy, 61.5 per cent with desquamative and only 11.5 per cent with usual interstitial pneumonia improved, whereas 27.0 per cent and 69.2 per cent worsened. We conclude that the histologic classification of chronic interstitial pneumonia used here permits forecasts of prognosis and response to treatment that cannot be deduced from other data.