We studied the clinical significance of genomic amplification of N-myc in Stage IV-S neuroblastoma, with reference to spontaneous regression. Among 103 neuroblastomas in which N-myc was measured, ten were Stage IV-S (eight children were younger than and two were older than 1 year of age). The number of copies of N-myc was 1 to 3 in five patients, four to ten in one patient, and more than ten in four patients, and the survivors of each group were four, one, and one (recurrent), respectively. Of 41 patients younger than 1 year of age, N-myc amplification of more than three copies was found only in Stage IV-S neuroblastoma. Cure with a tendency to regress spontaneously was seen in five of eight patients younger than 1 year of age. However, two patients older than 1 year of age classified as Stage IV-S (one with N-myc amplification) died of progressive disease. In two patients (1 and 3 months of age) with a huge hepatic involvement and in whom the tumor had an amplified N-myc of more than ten copies, tumor regression occurred but there was a relapse to a progressive state later. The overexpression of N-myc mRNA occurred in nine of ten stage IV-S tumors and did not correlate with the prognosis. The vanillylmandelic acid (VMA) to homovanillic acid (HVA) ratio was low in tumors with an increased number of copies of N-myc. Serum lactate dehydrogenase (LDH) levels were increased in Stage IV-S patients with N-myc amplification but not in those with regressing tumors and without N-myc amplification. These data suggest that N-myc amplification may affect the final outcome in the patient classified as Stage IV-S, but tumor regression can occur early after birth and appears to be independent of N-myc amplification.