The aim of this study was to assess the prevalence of hyperthyroidism and hypothyroidism in giant cell arteritis and polymyalgia rheumatica. The prevalence of thyroid dysfunction in giant cell arteritis and polymyalgia rheumatica patients was determined retrospectively from 1976 through 1984 and prospectively from 1984 through 1991. A control group was composed of patients over 55 years of age consecutively admitted to the same hospital department for another condition. Patients were screened for thyroid dysfunction using a thyrotropin assay. Abnormal results were evaluated by T3 and T4 assays and, if needed, a TRH test. Among the 68 giant cell arteritis patients (mean age 72.6 +/- 7 years), of which 41 were included in the prospective arm of the study, 6 had hypothyroidism and 3 had hyperthyroidism. Corresponding figures were 4 and 4 among the 36 patients with polymyalgia rheumatica (mean age 71.7 +/- 8.3 years), of which 18 were evaluated prospectively. Among the 305 controls (mean age 71.6 +/- 9.4 years), 16 had hypothyroidism and 10 had hyperthyroidism. Prevalences of hypothyroidism, hyperthyroidism, and antithyroid antibodies were not significantly different in the control and case groups. Data fail to support previous suggestions that giant cell arteritis or polymyalgia rheumatica patients may be an increased risk for hypothyroidism or hyperthyroidism. They lend no indirect support to the hypothesis that giant cell arteritis and polymyalgia rheumatica may be autoimmune disorders.