The short arm of chromosome 1p is the most frequently altered chromosome segment in neuroblastoma. The alterations, mainly deletions, are thought to be indicative of the presence of a tumor suppressor gene. To further refine the chromosome localization of this gene, we have studied paired constitutional and tumor DNA from a series of 60 patients with neuroblastoma at 2 minisatellite and 23 microsatellite loci dispersed along the short arm of chromosome 1. Twenty-two cases (37%) demonstrated loss of heterozygosity (LOH) at one or more loci on 1p. Surprisingly, the pattern of LOH enabled the identification of two distinct consensus regions of deletions. In agreement with previous reports, one region mapped to the distal short arm of chromosome 1. The other region was localized more proximally on 1p. Deletions observed in tumors involve either one or both of these regions. We show that the correlation between NMYC amplification and 1p deletion is limited to the deletions which involve the proximal region either alone or together with the distal region. These results suggest that two tumor suppressor genes on 1p might be involved in the development of neuroblastoma. Finally, we show that somatic mutations at microsatellite loci, frequently observed in other types of cancer, are rare events in neuroblastoma.