Genes in the HLA complex are by far the most important in determining genetic predisposition or resistance to Type 1 (insulin-dependent) diabetes mellitus. In this review evidence is presented that the HLA genes mainly involved are those encoding some particular HLA-DQ molecules. Both among Black, Caucasian and Japanese subjects particular cis or trans encoded DQ molecules are significantly associated with susceptibility, while others are associated with resistance. A varying degree of susceptibility or resistance seems to be conferred by these DQ molecules, where those determining resistance are dominant over those determining susceptibility. The degree of genetic predisposition to develop Type 1 diabetes carried by an individual would therefore be the result of his or her particular combination of DQ molecules. A primary association to particular DQ molecules explains previously found associations to other HLA complex genes by linkage disequilibrium. Some mechanisms by which particular DQ molecules may determine susceptibility or resistance are also discussed. Potential islet beta-cell reactive CD4+ T-cells may escape negative selection (deletion) in the thymus, but normally become anergized or remain ignorant extra-thymically. However, under particular circumstances they may be triggered. The DQ molecules associated with Type 1 diabetes susceptibility may preferentially bind and present triggering and/or beta-cell derived peptides to such T cells, causing beta-cell destruction. The finding that particular DQ molecules determine susceptibility may lead to new methods of preventing development of Type 1 diabetes in susceptible individuals.