Our aim was to determine whether the index of left ventricular mass increases during the acute and subacute phase of Kawasaki disease, and to investigate any relationship between this index and clinical and echocardiographic variables. We performed, therefore, a retrospective study of 66 children with Kawasaki disease, having a mean age of 2.85 years, comparing the findings with those obtained from 57 normal controls, having a mean age of 2.99 years. The data from the patients used for comparison was obtained during the acute and subacute phase of the illness, as well as during the phase of convalescence. We performed correlation analysis of the findings during the acute and subacute phase, determining the relationship between the index of left ventricular mass and other variables. The index was higher (p = 0.0461), and the velocity of propagation of left ventricular early diastolic flow was lower (p < 0.0001), during the acute and subacute phase when compared to control values. The index then reduced (p = 0.0001) during the phase of convalescent when compared to the acute and subacute phase. Levels of albumin in the serum (p = 0.0193), peak E velocity (p = 0.0479), and velocity of propagation (p = 0.0360) were found to be related to the index of left ventricular mass. Significant relationships were found between the differences in this index and differences in body weight when findings during the acute and subacute phase were compared to those of the phase of convalescence. The index of left ventricular mass, therefore, is increased during the acute and subacute phase of Kawasaki disease, and is associated with altered diastolic indexes. This elevation may be due to generalized myocardial swelling from acute inflammation and increased vascular permeability. Measuring this index as a potential predictor of diastolic function should be added to studies of cardiac function during the acute and subacute phase of Kawasaki disease.