The influence of left anterior hemiblock (LAH) on some electrocardiographic criteria of left ventricular hypertrophy (LVH) was investigated. 131 subjects were studied: 34 of them were normal (control group); 30 were carriers of isolated LAH; 38 were carriers of isolated LVH; 29 showed a pattern of combined LVH and LAH. The maximal voltages of QRS waves in leads usually analyzed for LVH, the QRS duration and the time of onset of the intrinsicoid deflection were compared among the groups. The specificity, the sensitivity and the performance of some LVH criteria were examined as well. LAH has been shown to conceal LVH on precordial leads, while it tends to make LVH more manifest on limb leads. The Romhilt and Estes' point score system proved to be the least affected by LAH. On the grounds of theoretical considerations and of empirical trials, we suggest modifying the Romhilt and Estes' point score system; furthermore we suggest modifying Sokolow's and Grant's criteria only when LAH is present. Such modified criteria have improved the accuracy of LVH prevision.