Many other autoimmune and chronic diseases exhibit marked geographic variation in incidence, which has been attributed to environmental differences across populations (Hutt and Burkitt, 1986). The results of our international IDDM research have provided evidence for the importance of large genetic variations in the frequency of HLA susceptibility genes between racial groups and countries. One may speculate that differences in the prevalence of susceptibility genes for other chronic diseases exist and significantly contribute to the geographic patterns of incidence of these disorders. Other autoimmune diseases are known to have epidemiological features similar to those described for IDDM. Although they are also characterized by an underlying HLA-related susceptibility, environmental factors are known to play an important aetiological role (Tiwari and Terasaki, 1985). DNA polymorphisms of the DR, DQ and DP locus antigens are associated with various autoimmune diseases (Todd et al, 1988; Thorsby et al, 1989). These molecular variations are similar to those described for IDDM, in that they are typically related to the hypervariable regions of the molecule and, thus, affect the peptide binding ability of the antigen. Based on the evidence for IDDM, population differences in the frequency of other HLA susceptibility genes are likely to be major determinants of the geographic distribution of diseases such as rheumatoid arthritis and multiple sclerosis. The epidemiological approach outlined in this review is, thus, applicable to other autoimmune diseases and will significantly contribute to our knowledge of the aetiology of these disorders. The emerging field of molecular epidemiology represents a new research approach which will lead to a better understanding of the relationships between specific risk factors and the aetiology of chronic diseases within populations and across the world.