We performed chromosome analysis of 15 neuroblastomas found by mass screening using a vanillymandelic acid spot test. We found near triploid chromosome abnormalities, ranging from 60 to 77 chromosomes, in the tumor cells from 14 patients, and hyperdiploidy with the mode of 50 in cells from one. A structural abnormality was observed in only one patient. We did not find a marker chromosome 1, homogeneously staining region, or double minutes, which have been previously reported in advanced neuroblastomas or in cell lines. All of our patients were completely free of disease 4 to 32 months after diagnosis. We consider that patients with hyperdiploidy or near triploidy are different from those with marker chromosome 1, homogeneously staining region, or double minutes and may constitute a subgroup with a good prognosis in childhood neuroblastoma.