Genomic amplification of the N-myc protooncogene in neuroblastomas correctly predicts poor outcome for the patients. However, the prognosis for neuroblastomas with a single copy of N-myc is also poor in cases diagnosed after 1 year of age but good in infantile cases. To elucidate the different prognoses depending upon the age of the patients with neuroblastoma, we performed an analysis of the expression of protooncogenes related to neural differentiation. We examined the genomic amplification of N-myc in 26 specimens of neuroblastomas and further analyzed 22 of the 26 cases for expression of N-myc, c-src, c-Ha-ras, and c-fos. Consequently, we observed frequent overexpression of N-myc in undifferentiated neuroblastomas and enhanced expression of c-src and c-Ha-ras in infantile neuroblastomas with favorable prognosis and in neuroblastomas differentiated by chemotherapy. These findings suggest that c-src and c-Ha-ras play important roles in the neural differentiation of infantile neuroblastomas.