Fragile sites on lymphocyte chromosomes of 20 patients with neuroblastoma of different clinical stages and histologic differentiation were studied. A hitherto unknown break in the band p13.1 of chromosome #1 was found in a frequency of 3%-15% in nine cases. The same phenomenon was observed in three members of a neuroblastoma family. In another instance, this fragile site was also seen in the mother of a patient. Fragile sites were expressed when cells were cultured in folate-deprived medium and could be slightly enhanced in frequency by aphidicolin. Additional aphidicolin induced possible fragile sites hitherto unknown or not yet accepted by the HGM 8 were detected.