The purpose of this study was to prospectively determine the incidence of diastolic mitral and tricuspid regurgitation in atrioventricular (AV) block using Doppler echocardiography. The temporal relation between mitral and tricuspid diastolic insufficiency and the diastolic murmur recorded in patients with complete heart block was also investigated. Twenty-two consecutive patients with AV block (referred to the Echo-Doppler laboratory for routine clinical studies), aged 18 to 87 years, were enrolled in the study. Eleven patients had third degree AV block and a ventricular-inhibited (VVI) pacemaker, two patients had second degree AV block, seven patients had first degree AV block, one patient had blocked premature atrial complexes and one patient had atrial flutter with 4:1 AV block. Diastolic mitral regurgitation was detected in 20 patients, and diastolic tricuspid regurgitation in 21. A mid-diastolic murmur was detected in all patients except in the three youngest. The murmur occurred before diastolic regurgitation and coincided with peak forward flow through the AV valve after atrial contraction. M-mode mitral valve echocardiograms obtained in nine patients demonstrated near closure of some portions of the mitral valve after atrial contraction. Effective closure of the valve, however, did not occur unless ventricular systole supervened. In conclusion, diastolic mitral and tricuspid regurgitation are almost universally present in patients with AV block and are associated with a diastolic murmur. The murmur coincides with forward AV valve flow. Diastolic regurgitation is silent. Effective AV valve closure is not established until ventricular systole occurs, as demonstrated by M-mode echocardiographic recording of the mitral valve.