Two of 200 patients treated with chlorozotocin have evidenced interstitial pulmonary disease. The cumulative doses of chlorozotocin were 480 and 670 mg/m2 when symptoms developed. Pulmonary function tests in both patients showed impaired diffusion, and an alveolar-arterial gradient which worsened with exercise. Open-lung biopsy in one patient showed typical interstitial inflammation and early fibrosis, and excluded lymphangitic spread of tumor. Discontinuation of chlorozotocin resulted in either improvement or stabilization of the pulmonary disease.