The genetic polymorphisms of the heat shock protein 70 (HSP70) and complement component C4 were investigated in 90 patients with hyperthyroid Graves' disease and 92 normal control subjects. The 8.5-kb PstI HSP70 allele was strongly associated with Graves' disease when compared with controls (P less than 0.001). The presence of the 8.5-kb PstI HSP70 allele was strongly associated with a deletion of the C4A gene in both patients and controls (P less than 0.0003 and P less than 0.00005 respectively). However, in the absence of C4A gene deletion, the frequency of the 8.5-kb PstI HSP70 allele was still significantly higher in patients when compared with controls (P less than 0.04). These results suggest that the HSP70 locus may have an immunological role to play in autoimmune hyperthyroid Graves' disease.