Congenital neuroblastoma and nesidioblastosis occurred simultaneously in a neonate with severe hypoglycemia. This combination may represent a rare type of complex neurocristopathy. Immunocytochemical staining revealed a diffuse proliferation of alpha, beta, and delta cells as clusters and individual cells. The total islet volume was less than control values and beta cell counts were not increased. This case tends to support the hypothesis that morphologic integrity of the normal islet architecture is important in the control of function and not merely excessive numbers of one cell type.