Computer processing of spatial QRS parameters (Frank vectorcardiogram) was used to study left posterior fascicular block. The best set of vectorcardiographic criteria was sought in order to characterise the classic pattern of left posterior fascicular block. Using these criteria, 18 cases were selected from a group of 4600 patients and 340 healthy subjects; isolated left posterior fascicular block was seen in 10 cases, and was associated with right bundle-branch block in 8 cases. It is shown that some external factors can influence the aspect of the spatial QRS loop in left posterior fascicular block: cardiomegaly tends to produce a superior displacement of the main QRS forces: right bundle-branch block enhances the superior displacement of the initial forces and shifts the main QRS forces more anteriorly and to the right. The 'masquerading effect' of the left posterior fascicular block on a concomitant inferior myocardial infarct was also shown. The most important diagnostic feature was the opposite direction of the initial forces (left anterosuperior) and the maximal vector (right postero-inferior): the angle between these two vectors averaged 152 degrees. Other criteria, such as the direction of rotation or the axis of the frontal loop, the vertical direction of the spatial loop, the presence of a Q wave in leads II, III, and aVF of the electrocardiogram, are not mandatory for the diagnosis of left posterior fascicular block.