The cases of two first cousins with neonatal diabetes mellitus are presented in this report. In one child the condition was transient, whereas the other has permanent diabetes mellitus. It seems unlikely that the difference in outcome was related to the degree of diabetic control early in the course of the disorder. The possibility of genetic influence in the etiology of neonatal diabetes is stressed. Chlorpropamide was not useful in curing neonatal diabetes mellitus in one case.