The onset of type 1A diabetes before the first year of age is a rare condition and is probably due to an interaction between genetic and environmental factors (infection), which, together, may explain such an early event. Studies say that about 15% of newly diagnosed type 1 diabetic patients had human Cytomegalovirus (CMV) specific viral genome in their lymphocytes. We report two cases of dizygotic twins with type 1 diabetes onset in their first 9 months of age, with genetic homogeneity (for HLA DR3/DR4 alleles), a history of CMV infection (positive IgG and urinary PCR) and positive antibody anti-GAD (9.6 UI/ml), present only in the second twin. Although they were dizygotic twins, which concordance rate is 3.8%, they assume the equivalent risk as monozygotic (40%) as they have similar high risk genotype (HLA) for type 1 diabetes. We believe that both time concordance and also the early onset of diabetes are due to an association between infection and the high genetic liability.