N-myc gene amplification is an unfavorable prognostic factor in neuroblastoma; therefore, its detection might have therapeutical consequences. Because of the reliability of noninvasive diagnostic methods such as X-ray examination and measurement of vanilymandelic acid and the lack of technical unavailability at most institutions, the determination of the N-myc status of neuroblastoma often is not done. In our investigation of 14 neuroblastoma patients, we could demonstrate N-myc gene amplification in the bone marrow of 5 patients with neuroblastoma stage IV disease and in bone marrow infiltration without enrichment of neuroblastoma cells. Otherwise, no information about the tumor content of N-myc gene copies at the time of initial diagnosis could be obtained. At the subsequent resection, the N-myc gene amplification was confirmed by the additional Southern blot analysis and in situ hybridization of tumor tissue. Furthermore, the N-myc status of bone marrow was analyzed during the stages of chemotherapy in three cases.