To measure the kidney size in children with Kawasaki disease (KD) and to delineate the condition of nephromegaly, 20 children with KD were enrolled in our study. Kidney sizes were measured during acute stage in these patients. Twenty healthy children and 15 febrile children served as healthy controls and fever controls, respectively. To delineate the possible mechanism, we also evaluated kidney volume (KV) in 13 other patients with KD during the acute phase and after the recovery phase as well as 26 healthy children. Plasma hepatocyte growth factor (HGF) and transforming growth factor-beta1 (TGF-beta1) levels were determined for all children. Kidney lengths and KV in patients with KD during the acute phase were significantly larger than those of healthy children. There was no kidney enlargement in healthy controls and after the recovery phase. The ratio of patient plasma HGF/TGF-beta1 during the acute phase and after the recovery phase correlated positively with the degree of nephromegaly in all patients. These results confirm the presence of large kidneys in children with KD. Our data also suggest that an elevated HGF/TGF-beta1 ratio may be responsible for the transient nephromegaly in these children.