Twenty-seven infants under one year of age with metastatic neuroblastoma were studied for osteomedullary metastases. They were evaluated by at least two imaging procedures: X-ray films and MIBG scan. Taking into account the results of these investigations, 3 groups were defined: osteomedullary metastases were detected in 8 infants by both X-ray and MIBG scan, no osteomedullary sites were detected by either technique in 13 patients, and 6 had osteomedullary foci with positive MIBG uptake but no radiological abnormality. These three groups were apparently different in terms of median age, response to chemotherapy, and long-term survival. The third group, which has not been previously described, appears to have a better prognosis than patients with radiologically detectable bone lesions. Nevertheless it was necessary to use intensive chemotherapy to obtain remission in these particular patients. It should be mandatory to specify the investigatory technique used to describe osteomedullary metastases in infants and treatment should be adapted according to this grouping system.