A genetic predisposition associated with HLA B27 for developing complete heart block with or without clinical or radiological signs of associated rheumatic disease has recently been found. In this electrophysiological study of 12 patients with spontaneous complete heart block and HLA B27 associated disease, of whom eight had ankylosing spondylitis, 10 had suprahisian second or third degree atrioventricular block (eight spontaneously and two during atrial pacing at rates below 90 impulses per minute) and one infrahisian block. One patient with narrow QRS complexes during complete heart block three months earlier had normal findings. Three patients also had sinus node malfunction and six had fascicular or bundle branch block. In HLA B27 associated disease the atrioventricular block seems to be preferentially located in the atrioventricular node, although the conduction system may be widely affected. The findings in this study indicate a further cause of high degree atrioventricular block with a predominantly suprahisian location in addition to acute inferior myocardial infarction, digitalis intoxication, and "congenital" heart block.