A 15-fold increase in the prevalence of ankylosing spondylitis in a group of 223 men with permanent cardiac pacemakers has recently been demonstrated. In this study of the same patient group, the prevalence of other related rheumatic syndromes was investigated by clinical examination and HLA typing. The clinical picture and electrocardiographic features of all patients with HLA B27-associated rheumatic disease (seronegative spondarthritis) were analyzed. Altogether 28 patients, 12.6 percent (95 percent confidence limits: 8.2 to 17.0 percent), fulfilled inclusion criteria for seronegative spondarthritides; 15 had ankylosing spondylitis. The seronegative spondarthritides previously had been diagnosed in less than 50 percent of the patients. Twenty-two (85 percent) of the 26 HLA-typed patients were B27-positive, implying a very strong association with this genetically determined cell surface protein. Patients with severe bradyarrhythmias associated with the seronegative spondarthritides were thus found to constitute a large proportion of the 223 men with permanent pacemakers. A high frequency of aortic regurgitation and all kinds of bradyarrhythmias were found. Twenty patients had complete heart block--in a majority, occurring intermittently, but otherwise without distinguishing features.