Molecular and biological analyses of a neuroblastoma case in which the original tumor contained a nodular region are described. No significant difference was observed between the nodular and the surrounding tumor tissue with respect to histopathologic examination, N-myc amplification, and trkA expression. However, flow cytometric analysis demonstrated that the nodular region consisted of a hypertetraploid clone, whereas the surrounding tissue mostly contained a hyperdiploid clone. Chromosome analysis showed that each clone had a similar chromosome acquisition pattern, suggesting that the hypertetraploid cells of the nodular region arose from the hyperdiploid cells of the surrounding tissue. Moreover, primary culture findings of the tumor cells showed that the responses to nerve growth factor or retinoic acid were different between the two. Collectively, this case suggests the possibility that neuroblastoma acquires novel biological characteristics through karyotypic evolution in vivo.