A minority of patients with cryptogenic fibrosing alveolitis (idiopathic pulmonary fibrosis) respond to treatment with corticosteroids. The purpose of this study was to identify factors other than histologic features that relate to responsiveness and prognosis. An analysis of 120 patients on whom data were prospectively collected is reported. The relationships between pulmonary function measurements, differential cell counts on bronchoalveolar lavage fluid, physiologic response to treatment, and survival are discussed. Patients responding to treatment with corticosteroids or immunosuppressive drugs had more severe initial impairment of the FVC (p less than 0.0001) and a more recent onset of disease (p less than 0.001). Lavage cell counts showed that increased proportions of lymphocytes were associated with responsiveness to corticosteroids and good prognosis (p less than 0.001), whereas increased eosinophils or increased neutrophils without increased lymphocytes were associated with failure to respond (p less than 0.025), and increased eosinophils were associated with a greater likelihood or progressive deterioration (p less than 0.05). Life table comparisons of survival in different groups showed improved survival in patients responding to treatment (p less than 0.05). Among nonresponders, those with more severe impairment of the FVC had shorter survival (p less than 0.05). We concluded that physiologic and bronchoalveolar lavage data allow predictions of responsiveness to treatment and prognosis that are of practical value to clinicians.