To examine the usefulness of the surface electrocardiogram in predicting left ventricular wall thickness as determined with echocardiography, standard echocardiograms, electrocardiograms and Frank lead vectorcardiograms were obtained in 30 volunteers. End-diastolic thickness of the interventricular septum and free posterior wall was measured from the echocardiogram and compared with the sum of the S wave in lead V1 plus the R wave in lead V6 (VS1+R6) and the magnitude of the Frank lead vector (Vf), a scalar dunction obtained from a simple analog device. The maximum of Vf, the summated vector (Vf), was highly correlated with VS1+R6 (r=0.84). There was significant correlation between the summated vector and VS1+R6 and the thickness of the interventricular septum (IVS) (r=0.73 and 0.66, respectively). The best least mean square fit for the population was Vf=1.7 IVS-0.39. There was no significant correlation between these variables and the end-diastolic thickness of the posterior wall. Volunteers who were athletically inclined or were joggers tended to have larger summated vector values and evidence of symmetric or asymmetric left ventricular hypertrophy in the echocardiogram. It therefore appears that the thickness of the interventricular septum has a greater influence on the summated vector and VS1+R6 that the echocardiographically assessed thickness of the free posterior wall of the left ventricle. The implications of these findings in the light of recent reports about the incidence of echocardiographically diagnosed left ventricular hypertrophy are discussed.