Background:There are different risk factors that affect the intensive care unit (ICU) stay after cardiac surgery.Objectives:The aim of this study was to evaluate possible risk factors influencing prolonged ICU stay in a large referral hospital.Patients and Methods:We conducted a case-control study to determinate causes of prolonged ICU stay in 280 adult patients undergoing cardiac surgery in a tertiary care center for cardiovascular patients, Tehran, Iran. These patients were divided into two groups according to ICU stay ≤ 96 and > 96 hours. We evaluated perioperative risk factors of ICU stay > 96 hours.Results:Among the 280 patients studied, 184 (65.7%) had stayed ≤ 96 hours and 96 (34.3%) had stayed > 96 hours in ICU. Frequency of prolonged ICU stay was 34.2% in patients undergoing coronary artery bypass graft (CABG), 30.8% in patients with valve surgery, and 44.8% in patients with CABG plus valve surgery. Patients with > 96 hours of ICU stay received more blood transfusion and intravenous inotropes. They also had longer anesthesia, cardiopulmonary bypass, and postoperative intubation time. There were higher incidence of postoperative tamponade, re-exploration, re-intubation, hemodialysis, and hypotension in this group (P < 0.05 for all comparisons).Conclusions:In this study, about one-third of patients had prolonged ICU stay. Factors influencing prolonged ICU stay were medical and some non-medical factors. In the present study, up to 30% of the patients had a prolonged ICU stay of > 96 hours. Additional data from well-designed investigations are needed for further assessment of the factors influencing prolonged ICU stay after cardiac surgery.