Tolvaptan, an arginine vasopressin V2 receptor antagonist, is considered to be an important drug for treating heart failure in children; however, there are few reports about its use in such cases. Herein, we describe our retrospective investigation into the efficacy and safety of tolvaptan in infants. This study examined 5 pediatric heart failure cases involving children with congenital heart disease (CHD) who were admitted to the ICU during the perioperative period or due to a history of CHD. The children were administered tolvaptan as a treatment for water retention after other diuretic drugs had proved to be ineffective. Improvements in their short-term clinical symptoms, such as weight reduction and increased urine volume, were observed after the administration of tolvaptan. The dosage ranged from 0.1-0.3 (median 0.1) mg/kg/day, which was lower than the doses used in previous studies, suggesting that tolvaptan is effective at low doses.