The presence and distribution of N-myc gene product were studied in 13 neuroblastomas and five ganglioneuroblastomas, using immunohistochemical techniques. Nine tumors (eight neuroblastomas and one ganglioneuroblastoma of composite type) contained neuroblastoma cells with positive nuclei for N-myc protein. Microscopic examination showed that most of the positive neuroblastoma cells seemed to be immature, with no apparent neuronal differentiation. Nine of 11 tumours with amplified N-myc gene copies exhibited tumour cells with positive immunostaining for the N-myc gene product, while none of the seven non-N-myc amplified cases contained immunoreactive tumour cells. The survival of the patients positive for N-myc protein was significantly low compared with that of the negative ones. It is concluded that immunohistochemical staining for the N-myc gene product will facilitate prediction of the prognosis of patients with neuroblastoma.