There is abundant evidence for a genetic influence on the development of autoimmune thyroid diseases (AITD). One measure of the magnitude of genetic contribution to the development of a disease is the sibling risk ratio (λs). Recent accurate prevalence data for hypothyroidism and hyperthyroidism in the United States reported from the National Health and Nutrition Examination Survey III (NHANES III) study have now allowed us to compute the sibling recurrence risk for AITD. Patients were recruited from our endocrine clinic on the basis of having AITD. The inclusion of patients in this study was unambiguously single ascertainment. We studied 155 patients (131 with Graves' disease [GD] and 24 with Hashimoto's thyroiditis [HT]) who had reliable information on the presence or absence of AITD in siblings. Nine probands had siblings with GD and 13 probands had siblings with HT. Using the prevalence rates from NHANES III for clinical hyperthyroidism and hypothyroidism, the calculated λs was 16.9 for AITD, 11.6 for GD, and 28.0 for HT. These results confirm the significant contribution of genetic factors to the development of AITD.