Sixteen patients with untreated small cell carcinoma of the bronchus received cyclophosphamide in a total dose of 160-200 mg/kg. Autologous marrow transplantation was used to minimise the period of hypoplasia and 2-mercaptoethane sulphonate to prevent urothelial toxicity. The procedure was well tolerated, with predictable and manageable toxicity. Complete radiological and bronchoscopic response was achieved in seven patients and partial response in a further seven. High-dose cyclophosphamide may be a useful initial treatment for this disease.