['To assess the impact of an automated intraoperative alert to redose prophylactic antibiotics in prolonged cardiac operations.', 'Randomized, controlled, evaluator-blinded trial.', 'University-affiliated hospital.', 'Patients undergoing cardiac surgery that lasted more than 4 hours after the preoperative administration of cefazolin, unless they were receiving therapeutic antibiotics at the time of surgery.', 'Randomization to an audible and visual reminder on the operating room computer console at 225 minutes after the administration of preoperative antibiotics (reminder group, n = 137) or control (n = 136). After another 30 minutes, the circulating nurse was required to indicate whether a follow-up dose of antibiotics had been administered.', 'Intraoperative redosing was significantly more frequent in the reminder group (93 of 137; 68%) than in the control group (55 of 136; 40%) (adjusted odds ratio, 3.31; 95% confidence interval, 1.97 to 5.56; P < .0001). The impact of the reminder was even greater when compared with the 6 months preceding the study period (129 of 480; 27%; P < .001), suggesting some spillover effect on the control group. Redosing was formally declined for 19 of the 44 patients in the reminder group without redosing. The rate of surgical-site infection in the reminder group (5 of 137; 4%) was similar to that in the control group (8 of 136; 6%; P = .42), but significantly lower than that in the pre-study period (48 of 480; 10%; P = .02).'] Does improvement of intraoperative antibiotic prophylaxis in prolonged cardiac surgery by automated alert in the operating room?