A 73-year-old man was admitted on April 1984 because of an abnormal shadow on chest X-ray. He was diagnosed as having small cell lung carcinoma (oat cell type) with inappropriate secretion of antidiuretic hormone (SIADH). Initial chemotherapy with cyclophosphamide, ACNU, vincristine, adriamycin, cis-platinum and etoposide was administered. The mass subsequently disappeared, and the serum sodium level normalized. About 2 years later, the patient developed a relapse of the primary lesion with hyponatremia. The same regimen as the initial chemotherapy was initiated and a complete response was again obtained. Similar episodes were repeated four times, and he died in April 1991. This patient survived for about seven years after the initial treatment without maintenance chemotherapy.