Six patients with breast cancer developed pulmonary toxicity following mitomycin therapy. The presenting symptoms were shortness of breath and a dry cough. The radiological pictures varied from a normal chest x-ray to extensive bilateral pneumonitis. The histological findings consisted of diffuse alveolar damage progressing to interstitial pulmonary fibrosis. Corticosteroid therapy resulted in complete resolution of pneumonia in one patient. Respiratory symptoms improved in 3 patients following discontinuation of the drug. Two patients with extensive bilateral pneumonitis, who were not treated with steroids, died of respiratory failure. Mitomycin-induced lung toxicity appears to be reversible with the discontinuation of drug and the administration of corticosteroids. Lung biopsy is necessary in order to rule out other diagnoses.