The prevalence of thyroid dysfunction and thyroid antibodies was investigated in a small rural community (Naerøy) and in Oslo. Attendance rates in Naerøy and Oslo were 99 and 71%, respectively. The prevalence of undiagnosed latent hypothyroidism and primary hypothyroidism was 4.0 and 5.3% in women in Oslo and Naerøy respectively and 0 and 3.5% in men. Undiagnosed hyperthyroidism was detected in 1.2% of men in Naerøy and in 1% of women in Oslo. Antibody to the thyroid microsomal antigen in titre greater than or equal to 400 was detected in 10.2 and 17.5% of women and 7.3 and 7.2% of men in Oslo and Naerøy, respectively. Among women with antibody to the thyroid microsomal antigen in both Oslo and Naerøy, and among men in Naerøy, the prevalence of past or present thyroid dysfunction was increased.