Introduction: Informed consent constitutes one of the most important legal, professional, and ethical principles of a surgical operation. Consent obtained from a patient is only valid when the patient has received enough information regarding the proposed treatment option. This study aims to determine how much the patients are informed before undergoing surgery, as well as the factors influencing it in hospitals affiliated with Tehran University of Medical Sciences. Method: This is a cross-sectional, descriptive–analytic study of 300 patients undergoing surgery in (...) 7 teaching hospitals affiliated with Tehran University of Medical Sciences. The patients were recruited through clustered sampling. Data were collected using a questionnaire completed by interview. Data were analyzed on SPSS software using descriptive and inferential statistics. Result: The mean score of data provision for patients was 27.09 out of 60, indicating the level of information provided as unacceptable. Among 12 questions dealing with data provision, patients had received an intermediate level of information about nature of the disease, type of surgery, benefits and importance of the surgery, and complications of rejecting the recommended therapy. On the contrary, they had not received enough information about the surgical procedure, type of anesthesia, potential complications of surgery, potential risks of surgery, other therapy options instead of surgery, length of hospital stay for surgery, postsurgical follow-up, and expenses of the surgery. In the majority (85%) of cases, the surgeon was reported to be the information provider. Among the variables studied, level of information received was directly related to the patient’s education level. Discussion: The findings of this study indicate that during the process of obtaining an informed consent for surgery, patients do not receive sufficient information, and it is necessary to provide the essential information in an understandable manner adjusted for the patient’s level of education. (shrink)