Chromosomal aberrations of 20 stage 4s neuroblastomas were analysed by comparative genomic hybridization (CGH). In a subset of 13/20 tumours, telomerase activity was evaluated by the telomeric repeat amplification protocol (TRAP). The CGH data were compared with the CGH results of ten stage 1 and 2 (stage 1/2) and 22 stage 3 and 4 (stage 3/4) neuroblastomas. A total of 17/20 stage 4s neuroblastomas did not progress clinically, whereas tumour progression with lethal outcome occurred in 3/20 cases. The CGH data of clinically non-progressing stage 4s tumours revealed a high rate of whole-chromosome aberrations (73.4%) with an overrepresentation of mainly chromosomes 2, 6, 7, 12, 13, 17, 18 and an underrepresentation of mainly chromosomes 3, 4, 11, 14. MYCN amplification or 1p deletion was observed in only 1/27 or 2/17 clinically non-progressing stage 4s tumours respectively, whereas all three progressive stage 4s neuroblastomas showed MYCN amplification, 1p deletion and, in 2/3 cases, distal 17q gains. Except for one case, telomerase activity was not observed in non-progressing stage 4s neuroblastomas. In contrast, 4s tumours with lethal outcome revealed elevated telomerase activity levels. Our data suggest that stage 4s neuroblastomas belong to two biologically different groups, one of which displays the genetic features of localized stage 1/2 tumours, whereas the other mimics advanced stage 3/4 neuroblastomas.