Immunocytologic analyses of bone marrow can provide clinically useful prognostic information in neuroblastoma. While analyzing the bone marrow with a panel of monoclonal antibodies, which detect neuroblasts and other defining B-, T-, and myloid lineage, we identified two infants with stage IV-S neuroblastoma whose bone marrow contained a large population of common acute lymphoblastic leukemia (ALL)-like cells. This population expressed HLA-DR, CD19(B1), CD10(CALLA), and occasionally CD20(B1). Since 1988, 17 additional patients with advanced neuroblastoma (IV-S, III, and IV) were studied by us. In 10 of the 19 patients, the bone marrow revealed an expanded CD10 population (20-70%). It appears that this group of patients has a better prognosis. Out of 9 patients who did not have an expanded CD10 population, 8 died within 9 months from diagnosis, whereas out of 10 patients with an expanded CD10 population only one died and the others are alive, 6-30 months from diagnosis (P < 0.001). An expanded CD10 population in the bone marrow of disseminated neuroblastoma patients may therefore serve as a prognostic factor. Apart from the prognostic value of this particular population in the single patient, its presence may shed light on the interrelationship between the immune system and the neuroendocrine compartment.