The association between rheumatological and thyroid disorders has long been known, the most common being the association of rheumatoid arthritis and autoimmune thyroiditis. Little is known as to possible thyroid involvement in other rheumatological disease of possible autoimmune aetiology, such as psoriatic arthritis and ankylosing spondylitis. We measured thyroid volume and function as well as the prevalence of anti-microsome and anti-thyroglobulin antibodies in 107 consecutive patients with rheumatoid arthritis, 42 patients with psoriatic arthritis, and 12 male patients with ankylosing spondylitis. Fifty-two normal subjects were used as controls. The average thyroid volume, measured at ultrasounds, was increased in all groups of patients, and the prevalence of thyroid enlargement (A-P diameter > 20 mm) was 2-3 fold higher in rheumatological disorders in comparison to controls. Both, patients with rheumatoid arthritis and psoriatic arthritis had higher-than-normal fT4 levels and an increased prevalence of anti-microsome antibodies. In the rheumatoid arthritis group alterations in thyroid volume and function were present irrespective of disease activity, whereas in psoriatic arthritis thyroid involvement was confined to patients with active disease. Our data are consistent with a significant thyroid involvement in rheumatological disorders, which is not limited to diseases with a definite autoimmune aetiology.