The relationship between the HLA system and insulin-dependent diabetes mellitus (IDDM) is reviewed. Data compiled by the HLA and Disease Registry reveal that HLA-B8 and/or Dw3 are associated with IDDM in all populations studied so far, but further population studies in non-Caucasian populations should be performed. In Caucasians, HLA-Dw2 renders protection against IDDM while HLA-Dw3 and Dw4 are associated with susceptibility to IDDM. The exact mode of inheritance of susceptibility to IDDM remains to be established. Involvement of at least two genes is likely. Heterogeneity of IDDM is highly possible and should be a matter of major interest in diabetes research.