To examine the nature of the first heart sound in left bundle branch block (LBBB), 26 patients with LBBB were studied using echophonocardiography. Apexcardiograms and systolic time intervals were also obtained from these patients and the results were compared with 20 normal subjects. Mitral closure preceded tricuspid closure by 0.02-0.04 second in 19 of the 20 normal subjects. In contrast, in subjects with LBBB there was a variable delay in mitral closure so that in seven subjects the sequence of mitral and tricuspid closure was normal, in 11 it was reversed compared wih the normal group, and in eight closure of both valves was simultaneous. The interval from the onset of ventricular depolarization to mitral closure was 0.074 +/- 0.019 (mean +/- SD) for the patients with LBBB and 0.057 +/- 0.008 second for the normal group (p < 0.001). With LBBB the first sound tended to be of reduced intensity and separate mitral and tricuspid components could not be identified. Systolic time intervals and the apexcardiograms demonstrated a delayed onset of left ventricular contraction and an increased isometric contraction time in the LBBB group, both of which would contribute to a delayed mitral valve closure and decreased first heart sound intensity.