Serial His bundle recordings were obtained during 1:1 atrioventricular (AV) conduction in 90 patients with chronic bundle branch block over a mean interval of 30 months. Atrioventricular conduction time (AH) increased greater than or equal to 10 msec in 25 (28%) and infranodal conduction time (HV) increased greater than or equal to 8 msec in 29 (32%), but only 10 patients had parallel increases in AH and HV intervals. Increases in conduction times were independent of age, time interval between studies, cause of heart disease or initial AH or HV intervals. Women were significantly more likely than men to show an increased HV interval and spontaneous trifascicular block. Spontaneous progression to second- or third-degree AV block occurred at the AV node in seven patients and below the node in 12 patients. The initial AH interval was prolonged in five of seven patients (71%) with AV nodal block and had increased further in only two at restudy. The initial HV interval was abnormal in eight of 12 patients (67%) who progressed to infranodal block and was prolonged further in eight at restudy. We conclude that in patients with chronic bundle branch block, (1) approximately 33% show progressive AV conduction system disease and AV nodal and infranodal disease progress independently; (2) progression of infranodal disease is more common in women; (3) AV nodal disease progress independently; (2) progression of infranodal disease is more common in women; (3) AV nodal disease is a common cause of AV block and can occur without further prolongation of the AH interval once a critical level of disease is attained, whereas infranodal block is usually accompanied by progressive lengthening of the HV interval; and (4) progression of AV conduction disease is not readily predictable from clinical and electrophysiologic variables.