Patients undergoing heart surgery are at high risk of postoperative fluid accumulation due to long procedures and cardiopulmonary bypass. In the present study, we sought to investigate the prevalence of postoperative fluid accumulation and its relation to adverse events in patients undergoing cardiac surgery. CAREBANK is prospective, single-center cohort study focusing on the adverse events after cardiac surgery. The study population was divided into 2 groups based on 5% postoperative weight gain. All the in-hospital adverse events are registered on the database. The end points of the present study were length of hospital stay, length of intensive care unit stay, occurrence of new-onset atrial fibrillation after hospital major bleeding episodes major cardiac events, cerebrovascular events, and death. Three-month and 1-year follow-up data also include all major adverse events. Altogether 1001 adult cardiac surgery patients were enrolled. The most frequent operations were coronary artery bypass grafting (56.3%). Five hundred fifty-four out of 939 (59.0%) patients had ≥5% weight gain during index hospitalization. Patients with a weight gain ≥5% were more likely to be women, have lower body mass index, had heart failure, and more often had preoperative atrial fibrillation. In-hospital period fluid accumulation was associated with reoperation due bleeding and longer total hospital stay. At 3 months' follow-up, weight gain 5% or more was associated with increased occurrence of new-onset atrial fibrillation, this was not reflected in the occurrence of strokes, transient ischemic attacks, or myocardial infarctions. Postoperative fluid excess is associated with adverse outcomes in cardiac surgery. Women, low-weight patients, and patients with cardiac failure or atrial fibrillation are prone to perioperative fluid accumulation.