Previous studies have pointed to many different prognostic factors for small cell lung cancer (SCLC) but diabetes mellitus (DM) has not been clearly or consistently identified as of prognostic value. The aim of this study was to investigate the prognostic significance of the characteristics of patients and clinical laboratory tests in SCLC. Specifically, we investigated that the impact of DM for survival in the patients receiving first-line etoposide plus cisplatin (EP) chemotherapy.