The prevalence of primary Sjögren's syndrome (primary SS) among patients with autoimmune thyroiditis (AT), and that of AT among patients with primary SS were studied prospectively. Of 63 patients with AT, one had precipitating antibodies against SS-A/Ro antigen together with objectively verified primary SS, and none had precipitating anti-SS-B/La antibodies; in contrast 17/63 (27%) had above normal values of anti-SS-B/La antibodies, as tested with ELISA. Of 19 AT patients tested objectively for xerostomia and keratoconjunctivitis sicca, six (32%) had keratoconjunctivitis sicca together with xerostomia and four (21%) had autoimmune sialadenitis. The prevalence of AT in patients with primary SS (n = 28) was 18%; of the 28 patients, 64% had an enlarged or abnormal thyroid gland on palpation and four of them (14%) had cytology verified AT. Ten (36%) had anti-thyroglobulin and/or antimicrosomal autoantibodies. Based upon the present investigation we conclude that the prevalence of primary SS is ten times higher among patients with AT, and that of AT is nine times higher among patients with primary SS, compared with the general population.