To assess the usefulness of bone marrow (BM) examination performed as part of the staging procedure in SCLC, 137 patients with this neoplasm were studied. At diagnosis, BM was infiltrated in 23 patients (17%) and in 5 of them (22%) it was the only metastatic site. BM examination changed the previously given stage in 9% of the patients who were supposed to have limited disease before this test was performed. BM involvement correlated with a higher frequency of bone (p less than 0.05) and liver (p less than 0.01) metastasis. Correlations were found neither with blood or serum biochemistry disturbances nor with postchemotherapy haematologic toxicity. The BM positive patients had a survival rate similar to those with extensive disease but without BM infiltration. We conclude that BM examination should be performed only when no other metastatic sites have been found after a complete staging study.