Neuroblastoma shows remarkable heterogeneity, ranging from spontaneous regression to progression toward highly malignant tumors. In search of genetic abnormalities that could explain this variability, we have characterized neuroblastoma tumors by using multiple fluorescent hybridizations. Our results indicate that chromosome 17 is rearranged very frequently in the form of unbalanced translocations with numerous chromosomal partners, all leading to the presence of supernumerary copies of a 25 Mb chromosomal region originating from 17q23.1-qter. Additional 17q material was detected in more than 90% of untreated high-grade neuroblastomas and, along with 1p36 deletion, should represent the most frequent genetic abnormality of neuroblastoma observed until now.