The region on chromosome 6p21 (IDDM1) confers the largest part of genetic susceptibility to type 1 diabetes (T1D) with particular human leucocyte antigen (HLA) alleles predisposing and others protecting from it. As T1D is primarily a "sporadic" disease, the pathophysiology must involve gene-environment interactions. We searched for indirect evidence for such major histocompatibility complex (MHC)-environment interactions by asking two questions: (i) can the degree of an HLA association vary over time periods? and (ii) if a prenatal event like an intrauterine infection - that might cluster in seasons - leads to differences of HLA associations in patients with particular birth months?