Mild aortic root dilatation, cusp thickening and subvalvular fibrous ridges have been reported as characteristic in patients with ankylosing spondylitis and aortic regurgitation. Thirty-five patients with ankylosing spondylitis (10 also had Reiter's syndrome) without clinically apparent cardiac involvement were studied using phased array two dimensional and sector-directed M mode echocardiography to determine the prevalence of aortic abnormalities. Aortic root dimensions were measured at the aortic anulus, at the tip of the cusps and 0.5 to 1.5 cm above the cusps. The two dimensional echocardiographic study was also analyzed for qualitative abnormalities. The dimensions were compared with those in 20 normal men and among patient subgroups separated according to age, duration and severity of ankylosing spondylitis and presence of qualitative abnormalities. With one exception, no abnormally increased aortic dimensions suggestive of aortic dilatation were found in any group. However, two patients had aortic dimensions greater than 4.2 cm at the valve (normal 4.0 cm or less). Also, six patients had discrete areas of increased bright echoes below the left or noncoronary cusps suggestive of a subaortic "bump" and two of the six patients had increased aortic cusp echoes suggestive of thickening or fibrosis, or both. These changes tended to occur more commonly in older patients and those with more severe disease. It is concluded that aortic root changes suggestive of inflammation or fibrosis, or both, occur in asymptomatic patients with ankylosing spondylitis and are detectable on two dimensional echocardiography. Dilatation usually does not occur without aortic regurgitation.