An epidemiological investigation in 11 European countries comprising a total childhood population of 54.1 million children and using 8 separate data sources was conducted to evaluate the occurrence of neuroblastoma in Down syndrome (DS). No cases of DS were detected among 6724 infants and children with neuroblastoma, although more than five were expected. This highly significant result (P = 0.0045 according to the Poisson test) is consistent with data in the literature, which contains only two poorly detailed cases in epidemiological studies and one ganglioneuroma in a DS mosaic patient. Like other tumors, such as leukemias, testicular germ cell tumors and lymphomas are in excess in DS patients; the lack of neuroblastomas does not reflect a general decreased incidence of cancer but rather a specific underrepresentation of this precise tumor. S-100 b protein, the gene for which maps to the long arm of chromosome 21, (a) is overproduced in DS patients, (b) produces growth inhibition and differentiation of neural cells in vitro, (c) is abundant in good-prognosis neuroblastomas, and (d) has been shown to induce growth inhibition and differentiation and cell death in several human and murine neuroblastoma cell lines and could be responsible for this variation. Additional epidemiological and experimental studies are warranted to confirm our interpretation of these data.