Development of double cancer was evaluated in 337 small cell lung cancer patients who had received intensive chemotherapy with or without radiotherapy. Of them, 14 patients (4.2%) developed a second malignancy: non-small cell lung cancer in six, stomach cancer in four, acute myelogenous leukemia in two, liver cancer in one, and esophagus cancer in one. The relative risk for the development of double cancer calculated by person-year method utilizing age and sex adjusted cancer incidence in Japan was 2.75-fold (P<0.01). The risk of non-small cell lung cancer (8.75-fold) and acute myelogenous leukemia (37.82-fold) was particularly high. The cumulative risk for the development of double cancer was 2.0% at 1 year, 4.1% at 2 years, 14.3% at 3 years, and 100% at 8.1 years. Of 27 patients who survived disease-free for more than 2 years, 10 patients died; five patients (50%) died of double cancer, two died of infectious disease, and only three patients died from recurrent small cell lung cancer. These findings indicate that a cautious follow-up program for the detection of double cancer is indicated in patients with small cell lung cancer.