In a long-term culture of bone marrow taken from a child with a suspected abdominal tumor, a large number of globular in vitro clumps developed within 5 days, although the bone marrow used had tested negative to the presence of abnormal cells. Light microscopic and marker studies of these clumps provided evidence of neuroblastomal origin, and the diagnosis of a neuroblastoma was confirmed by a subsequent cathecholamine assessment and the histological findings of the excised tumor. Of three in vitro culture methods tested, i.e., a long-term bone marrow culture (LTBMC), a liquid culture, and a methylcellulose clonal cell culture, the LTBMC was found to be the most sensitive for detecting tumor cells. This LTBMC was considered to be useful for not only achieving a definitive neuroblastoma diagnosis but also the early detection of bone marrow metastasis in N-myc gene amplified neuroblastomas.