Of 129 patients with small cell lung cancer (SCLC) who underwent bone marrow examination for staging, 39 (30%) had bone marrow involvement. Only three of 129 patients (2.3%) had bone marrow involvement as the only site of metastatic disease. When patients with bone marrow metastasis were compared with patients whose bone marrow was normal, there were significant differences in serum levels of lactate dehydrogenase (LDH), glutamic oxalacetic transaminase (SGOT), glutamic pyruvic transaminase (SGPT), alkaline phosphatase (AP), albumin, and sodium (Na). We found no clinically significant difference in survival between patients with extensive disease with or without bone marrow involvement. Serum Na, albumin, SGOT, and uric acid were important prognostic determinants of survival. Based on the results of this study, we do not recommend routine bone marrow examinations in the staging of SCLC.