Graves' hyperthyroidism and dysthyroid eye disease are closely related autoimmune conditions. Whether the eye disease is an integral part of Graves' disease or a separate entity is controversial. To investigate this we have examined the genetic associations of ophthalmopathy and hyperthyroidism, and compared their phenotype and gene frequencies with a control normal population. HLA-A, B, and DR antigens were typed in 67 patients with dysthyroid eye disease (GO), 60 hyperthyroid patients without significant eye disease (HT) and 500 normal subjects. Patients were also typed for a variety of other genetic markers: blood group systems (10), serum proteins (6) and red cell enzyme systems (10). Increased frequency of B8 and DR3 in Graves' disease was confirmed; B17 occurred less frequently and appears to be protective. HLA antigen frequencies for GO did not differ from HT. The MNS blood group showed a significant association with Graves' disease, the HT patients having a deficit of the s gene compared with controls. The most interesting finding was an increased frequency of blood group P in GO patients compared with either HT or controls. Significant differences were not seen with any of the other HLA antigens, blood groups, protein or enzyme markers considered individually. Multivariate analysis applied first to the HLA and then to the non-HLA systems indicated clear separation of the two patient groups. Although Graves' eye disease shares the same HLA associations as hyperthyroidism, it differs in the increased frequency of P blood group, suggesting that additional genetic factors may determine which patients with Graves' disease develop ophthalmopathy.