The purpose of this study was to assess the hypothesis that lower serum sodium levels are associated with cardiovascular sequelae in patients with Kawasaki disease (KD). We used the database of the 16th nationwide survey of KD in Japan. We investigated the distribution of serum sodium levels and the relationship between serum sodium levels and cardiovascular sequelae. Of the reported cases, serum sodium levels were reported in 13,569 patients (89%). The proportion of patients with serum sodium levels 130 mEq/L or less, was greater in complete cases than in incomplete cases. The proportion of patients with serum sodium levels 130 mEq/L or less was increased with age. The largest proportion of patients with serum sodium levels 130 mEq/L or less was found in the category of 3-5 days since onset of illness. A serum sodium of level 135 mEq/L or less was an independent risk factor for cardiovascular sequelae (odds ratio, 1.79, 95% confidence interval, 1.42-2.26). Among patients with KD, there are significant differences in serum sodium levels between diagnostic categories, age, and days since the onset of illness. The sodium level may be a simple predictor of cardiovascular sequelae.