One hundred and eleven unselected patients with hyperthyroidism due to Graves' disease received decreasing doses of carbimazole for 18 months. Clinical examination and hormonal assays (serum T3, T4, free T4 index) were done at 4, 9, and 18 months of treatment. Patients were typed for 35 HLA antigens and were followed for 2 yr after withdrawal of treatment; 39 patients were excluded for various reasons and 72 were retained for study. Of the 72 patients, 37 relapsed and 35 remained in remission; 40 patients were DR3+ (20 relapsed) and 32 were DR3- (17 relapsed). HLA frequency was not significantly different in patients who relapsed and those who remained in remission. Thus, under the conditions of this study, HLA frequency could not be used to predict relapse of hyperthyroidism due to Graves' disease.