A log-linear model was used to analyze three-way interactions between IDDM and pairs of genetic markers. To do this, a special sample dataset was selected by taking one affected and one unaffected child from each family. Some three-way interactions were found for associations between insulin-dependent diabetes mellitus (IDDM), HLA-DR, and DQ restriction enzyme fragment length polymorphism (RFLP) patterns. No three-way interaction was found for IDDM, HLA-DR, and Gm or Km. An extended sibpair analysis was applied to the HLA-B,DR loci and to the Gm, Km, and insulin gene polymorphisms. The well-known result for IDDM and HLA was reproduced. For Gm, Km, and the insulin gene no cosegregation with IDDM could be found.