Classical neonatal diabetes mellitus is defined as hyperglycaemia occurring within the first six weeks of life in term infants. Cerebellar agenesis is rare. We report three cases of neonatal diabetes mellitus, cerebellar hypoplasia/agenesis, and dysmorphism occurring within a highly consanguineous family. This constellation of abnormalities has not previously been described. Two of these cases are sisters and the third case is a female first cousin. The pattern of inheritance suggests this is a previously undescribed autosomal recessive disorder. Prenatal diagnosis of the condition in this family was possible by demonstration of the absence of the cerebellum and severe IUGR.