Twenty-six patients with small cell carcinoma of the lung were treated with a combination of methyl-CCNU (75 mg/m2 orally), cyclophosphamide (750 mg/m2iv), and vincristine (1.4 mg/m2iv) once every 3 weeks and followed for 2-56 weeks (mean, 24 weeks). An average of seven treatments were given per patient. Myelotoxicity was mild to moderate with no white blood cell count (wbc) less than 1000 cells/mm3 and no platelet count less than 25,000 cells/mm3. Six patients (23%) had a wbc of 1000-2000 cells/mm3 and two (8%) had a platelet count of 25,000-75,000 cells/mm3. Sixty-eight persons of the projected dose of methyl-CCNU was given. Fourteen of 22 patients with measurable disease (54%) responded. Of 14 patients who had received no prior treatment 64% responded with a median survival duration of 40 weeks. Complete responses occurred only in patients without prior radiation therapy or chemotherapy. We conclude that methyl-CCNU may be given with an acceptable level of toxicity in an every 3-week schedule and that the combination of cyclophosphamide, methyl-CCNU, and vincristine warrants further evaluation in the treatment of small cell carcinoma of the lung.