In a clinical and biochemical study of 559 subjects over the age of 65 years living in the community, in residential homes and in hospital, seven patients with thyroid disease were found. This gave a prevalence rate of 0.94% of hypothyroidism and 0.47% of hyperthyroidism in people aged 80 years or more. Five of these seven subjects were suspected clinically of having thyroid disease. Therefore routine biochemical screening does not appear to be justified. Within each of the age strata there was no significant change in serum thyroxine with age. Women had significantly higher serum thyroxine levels than men, in the group aged 75 years or more.