The electrocardiographic diagnosis of left ventricular hypertrophy is often difficult because it is based on a large number of criteria which, even if taken on their own or as "scoring systems", have a poor sensitivity ranging from 10 to 60%. Some authors have shown that the diagnosis is easier--though at first sight this seems paradoxical--in the presence of altered ventricular depolarization. To verify this statement in the case of left anterior fascicular block, we tested the value of six different indices for the detection of left ventricular hypertrophy. We analyzed 100 patients with left anterior fascicular block and compared the six electrocardiographic indices with the echocardiographic reference method, using the formula of the Penn convention, to establish the left ventricular mass. The best index of the six was that of Gertsch: [S3+ (R+S) maximal precordial] greater than or equal to 30 mm. Its sensitivity was 74%, its specificity 69%, its positive predictive value 79% and its negative predictive value 63%. The other tested indices had a sensitivity of less than 45% with a specificity of more than 80%. Furthermore, Gertsch's index was of equal value in confirming the increased left ventricular mass due either to concentric hypertrophy or to dilated cardiomyopathy. This study therefore confirms that electrocardiographic diagnosis of left ventricular hypertrophy is even easier in the presence of left anterior fascicular block than in absence of altered ventricular depolarization. This result is of practical interest, the incidence of left anterior fascicular block being 1 to 5% in the general population and as high as 30% after the age of 80.