One hundred cases of eosinophilic granuloma diagnosed by open lung biopsy were reviewed. There were 60 women and 40 men, ages 18 to over 60 years. Radiographs and clinical histories were contributed by physicians referring the cases for pathological consultation. Clinical follow-up information was obtained in 60 cases. The outcome was generally benign; the 16 asymptomatic patients remained well; 17 others had complete remission of symptoms, 22 had persistent symptoms, though half had partial improvement; 4 patients had progressive disease despite treatment, but only 1 patient died (of bilateral pneumothoraces complicating severe fibrosis). The more severe manifestations were found in young men, who had a higher incidence of pneumothorax, fibrosis and honeycombing, and diabetes insipidus. Many previously reported cases have had a less favorable outcome; milder cases of primary pulmonary histiocytosis have probably been overlooked in the past. In this series the prognostic value of histologic, clinical, or radiologic findings was limited. The diagnosis can often be strongly suggested radiologically, though infectious granulomata must be excluded; it is arguable whether exclusion of sarcoidosis is clinically important. The effectiveness of treatment with adrenal corticosteroids could not be assessed because of lack of controls; some individuals appeared to benefit, but relapse was very unusual in any case. The etiology of EG remains obscure, but in view of the paucity of patients with a history of allergy or asthma, and the lack of eosinophilia in any case, hypersensitivity seems unlikely. Smoking was far more common among these patients (97% altogether; 80% current) than in the general population (about 35%), an unexplained finding.