Aortic root abnormalities including cusp thickening, subvalvular stenosis, and mild aortic root dilatation are the most common cardiac complications in patients with long standing ankylosing spondylitis (AS). Twenty-three patients with definite idiopathic AS (New York Criteria 1966) and twenty-two matched controls were studied with M-mode echocardiography. Only one of the AS patients had clinical aortic incompetence. Six of the AS patients had mildly dilated aortic roots (normal less than 3.7 cm) with a mean diameter of 3.9 cm (range 3.8 to 4.00 cm). None of the twenty-two controls matched for age, sex and blood pressure had dilated aortic roots, with a mean diameter of 3.3 cm (range 2.9 to 3.6 cm). No correlation existed between aortic dilatation and severity of disease estimated by acute phase proteins--caerulo plasmin, alpha 1-antitrypsin, alpha 1 acid glycoprotein, ferritin and C Reactive protein. Contrary to a previous report, mild aortic root dilatation occurs in long standing cases of AS. Although it is a non-specific finding, it does not appear to be related to age or blood pressure and may therefore be the forerunner of aortic incompetence.