The vasopressin receptor 2 (V2) receptor antagonist tolvaptan is an aquaretic agent that has been found to improve symptoms in patients with congestive heart failure. In this study (SMILE study), we administered tolvaptan to patients aged ≥ 80 years with heart failure accompanied by congestive symptoms and compared its effectiveness and safety profiles in this group with those in patients < 80 years (U-80). The results showed that the effectiveness of tolvaptan in the aged patients was similar to that in U-80 patients. In the safety profile, the incidence rate of thirst was lower in the aged patients than that in U-80 patients (9.6% versus 11.6%, P = 0.0023). Furthermore, the incidence of hypernatremia, defined as ≥ 150 mEq/L in aged patients, was comparable with that in U-80 patients (2.9% versus 3.6%, respectively, P = 0.3657). Based on these findings, tolvaptan has similar effectiveness and safety profiles in aged patients compared with U-80 patients. In addition, we found that a higher starting dose of tolvaptan was markedly associated with the occurrence of hypernatremia exclusively in the aged population; therefore, we recommend that tolvaptan should be started at lower doses in aged patients.