Type 1 diabetes mellitus (T1DM) is an autoimmune disease characterized by autoimmune destruction of pancreatic beta cells. Genetic and environmental factors contribute in this disease. There is evidence that MHC class I chain-related gene (MIC-A) plays a role in the susceptibility to this and other autoimmune diseases. There are five alleles of the MIC-A gene, which consist of different repetitions of GCT. In particular, MIC-A alleles 5 and 5.1 (the former with five repetitions of GCT, the latter with five repetitions and one additional insertion of nucleotide G) have been found to be associated with susceptibility to and age at onset of T1DM. The aim of our study was to analyze the transmission of these MIC-A alleles to T1DM-affected offsprings in HBDI families. These are multiplex families with affected offsprings and unaffected parents. DNA samples were amplified for MIC-A using fluorescence-labeled primers and analyzed on an ABI prism DNA sequencer. The transmission of alleles was then analyzed using pedigrees of families also obtained from HBDI. We analyzed 78 families and found that MIC-A alleles 5 and 5.1 are present and transmitted more frequently than expected. Heterozygotic parents for MIC-A alleles 5 and 5.1 were excluded from the study. Our results suggest that MIC-A alleles 5 and 5.1 are associated with susceptibility to T1DM in family studies.