The cause of severe disturbances of the cardiac conduction system is seldom possible to establish clinically at pacemaker implantation, apart from cases of acute myocardial infarction or digitalis intoxication and in relatively rare cases of inflammatory disorders such as sarcoidosis and systemic sclerosis. Since cardiac manifestations, mainly conduction disturbances, occur in patients with ankylosing spondylitis, the prevalence of this disease was determined using radiologic screening for sacroiliitis in a population of 223 men who had permanently implanted pacemakers. Sacroiliitis was found in 19 men (8.5 percent), 15 of whom fulfilled the diagnostic criteria for ankylosing spondylitis. In six patients, sacroiliitis was asymptomatic and two of the patients were completely free of symptoms other than those originating from their heart manifestations. In seven of the 15 patients with ankylosing spondylitis and in the four patients with sacroiliitis without clinical criteria of ankylosing spondylitis, the diagnosis was previously unknown. Uveitis and aortic regurgitation occurred in five patients each, while peripheral arthritis was twice as common. The prevalence of sacroiliitis and ankylosing spondylitis of 8.5 and 6.7 percent, respectively, differ significantly (p less than 0.01) from the frequencies found in general Caucasian populations of 1 to 2 and 0.1 to 0.5 percent, respectively. HLA B27 was present in more than 80 percent of the patients with sacroiliitis and/or ankylosing spondylitis, compared with 8 to 10 percent in the general population. This strong association is in accordance with previous studies of patients with symptomatic sacroiliitis and/or ankylosing spondylitis. Thus sacroiliitis, diagnosed by x-ray, can be considered a marker for this relatively common rheumatic cause of severe disturbances of the cardiac conduction system.