Objective:The objective of the present study is to determine the incidence of delirium and the associated factors in patients undergoing open heart surgery. Methods:This is an Analytic-descriptive study conducted on 404 patients undergoing elective open heart surgery in Fatemeh Zahra Heart Center, Sari, over the period of 6 months from July to December 2011. Sampling was achieved in a nonrandomized targeted manner and delirium was assessed using NeeCham questionnaire. A trained nurse evaluated the patients for delirium and completed the risk factor checklist on days 1 to 5 after surgery. Data analyses were accomplished using survival analysis (Kaplan-Meier and Cox regression) on SPSS software version 15. Results:We found that variables, including ventilation time, increased drainage during the first 24 hours, the need for re-operation in the first 24 hours, dysrhythmias, use of inotropic agents, increased use of analgesics, increased arterial carbon dioxide, lack of visitors, and use of physical restrainers were associated with the development of delirium. In addition, we found a delirium incidence of 29%. Conclusion:Diagnosis of cognitive disorders is of utmost value; therefore, further studies are required to clarify the risk factors because controlling them will help prevent delirium.