Population and family studies show that predisposition to type I diabetes (IDDM) is multifactorial, and that polymorphisms in the MHC region contribute substantially to the susceptibility to IDDM. In the present study the association of polymorphisms in the CD4 and the delta subunit of CD3 with IDDM were examined in a Belgian population. We observed that the frequency of the CD A4/A4 genotype and of the CD3 91 allele were significantly increased P = 0.0077) and decreased (P = 3.8 x 10(-5), respectively, in IDDM compared with controls. These results therefore suggest that CD4, CD3 or neighbouring genes might contribute to IDDM susceptibility. These results are, however, preliminary and cannot be considered as established until re-tested in a new population.