OBJECTIVE--To assess the diastolic effect of left bundle branch block in patients with dilated cardiomyopathy. DESIGN--Retrospective study of M mode and Doppler echocardiograms along with electrocardiogram and phonocardiogram. SETTING--Tertiary referral cardiac centre. PATIENT PARTICIPANTS--Fifty two patients with dilated cardiomyopathy, all with functional mitral regurgitation. Twelve with left bundle branch block (group 1) were compared with 40 without (group 2). RESULTS--Mean (SD) age 60 (15) v 55 (18) years, left ventricular end diastolic dimension 72 (9) v 70 (7) mm, and heart rate 88 (15) v 84 (15) beats/min were similar in both groups. In patients with left bundle branch block the electromechanical delay, 50 (20) v 70 (20) ms, was shorter (p less than 0.05) whereas the preejection contraction time, measured from the onset of mitral regurgitation to that of aortic ejection, 130 (40) v 70 (20) ms (p less than 0.01), and left ventricular relaxation time, A2 to the end of mitral regurgitation, 130 (30) v 80 (30) ms (p less than 0.01), were both prolonged. Ejection time itself was similar, 230 (40) v 235 (40) ms. Thus the overall duration of mitral regurgitation was increased (495 (90) v 390 (60) ms (p less than 0.01], which made filling time shorter (190 (45) v 325 (90) ms (p less than 0.01]. In patients with left bundle branch block, unlike those without, contraction and relaxation times both shortened as RR interval fell; this made the duration of mitral regurgitation more sensitive to heart rate. Filling time was less than 200 ms in eight out of 12 patients with left bundle branch block and four out of 40 patients without (p less than 0.001). CONCLUSION--Left bundle branch block prolongs rather than delays mitral regurgitation by increasing pre-ejection and relaxation times. This directly impairs diastolic function by shortening the time available for the left ventricule to fill to an extent likely to limit stroke volume.