This study aimed to investigate left ventricular myocardial deformation in children with Kawasaki disease during the acute phase of their illness. A total of 50 patients and 35 normal control subjects were assessed. Data were obtained from the patients during the acute and convalescent phases of Kawasaki disease. Analyses of myocardial deformation [strain (ε), strain rate (SR)] was performed using two-dimensional speckle-tracking imaging in three directions (longitudinal, circumferential, and radial) at the basal and mid levels of the left ventricular myocardium. Basal longitudinal ε (P < 0.001) and midlongitudinal ε (P < 0.0001) were lower during the acute phase of the disease than in the control subjects and associated with serum albumin level and left ventricular mass index (LVMI). Midlongitudinal SR (P < 0.0001) was lower during the acute phase of Kawasaki disease than in the control subjects and associated with LVMI. Decreased systolic SR was not detected in any direction. In conclusion, left ventricular longitudinal systolic ε was significantly decreased during the acute phase of Kawasaki disease. This may be a result of myocardial swelling from myocarditis during the acute phase of the disease.