A 55-year-old woman with dilated cardiomyopathy and rate-dependent left bundle branch block had a cardiac resynchronisation therapy (CRT) device implanted. During implantation, the maximum rate of left ventricular pressure rise (dP/dt(max)) was measured invasively. This case presents a description of the acute negative effect of a left bundle branch block on dP/dt(max), and the different effect of CRT on left ventricular haemodynamic function in the presence and absence of a left bundle branch block.