From April 1982 to December 1987, 71 patients with small cell lung cancer entered a randomized clinical trial, and underwent bone marrow biopsy (BMB) as part of staging procedures. We identified 8 patients (11%) with bone marrow metastases, 6 with extensive disease independently of BMB, and 2 with extensive disease on the basis of the BMB only. BMB determined a change in the stage in only 3% (2/71) of the cases. No differences were found in the hematological parameters of the patients with or without bone marrow metastases. The median survival of the patients with bone marrow involvement was the same (41 weeks) as those with extensive disease but without bone marrow involvement. We conclude that unilateral BMB without aspiration detects a substantial proportion of bone marrow metastases in patients with extensive disease. This fact does not worsen the prognosis. A small proportion of patients with apparently limited disease has bone marrow involvement. The technique therefore contributes, to a small extent, to the definition of the clinical stage of the disease. However, bone marrow involvement is an important data of natural history, and therefore new methods to better assess this peculiar site of the disease are needed.