Two principal theories have been formulated to explain the simultaneous occurrence of a left bundle branch system block (LBBsB) and a stress-induced chest pain: initial stage of the permanent LBBsB of a primary myocardiopathy, or original clinical expression of coronary insufficiency. In the case presented here, the similarity of symptoms, the improvement observed in ergometric exercise tests under nitroglycerin and the increase of lactate concentrations in the coronary sinus were in favour of the second theory. However, the results of a radionuclide myocardial perfusion study and a radionuclide ventriculography were not sufficiently specific to support this assumption. The lack of free interval between the LBBsB and the angina-like chest pain (neurosensorial reflex) stands against an ischaemic mechanism. The initial intermittent LBBsB phase found in our patient during the exercise test might correspond to this free interval. The favourable prognosis and the uncertain effectiveness of treatment are suggestive of a very early stage in either primary cardiomyopathy or coronary disease.