Type 1 diabetes mellitus (T1DM) is an autoimmune disease in which the beta cells in the pancreas are destroyed by the body's own immune system. IL12 plays a role in pathological situations, such as septic shock, tissue damage during inflammation, and organ-specific autoimmune diseases. In NOD mice, administration of IL12 induces T1DM and administration of IL12 antagonists prevents T1DM. Linkage and association of IL12 to T1DM have been reported previously. We are unable to replicate this linkage on chromosome 5q33-34 in 184 Swedish families. Further, we exclude a gene with lambda(s) > 1.4 from this region. Together with previously published findings, these data make IL12 an unlikely major susceptibility gene for T1DM.