Several adverse effects of vancomycin have been reported. The aim of this study was to assess the incidence of adverse responses to antibiotic prophylaxis with vancomycin in cardiac surgical patients. Prospectively, 116 consecutive patients (106 adults and 10 children) undergoing cardiac surgical procedures in this institution from January to June 1990 were studied. After the anesthetic induction, vancomycin, 1 g in adults and 10 mg/kg in children, was intravenously administered over 30 minutes. The infusion rate was slowed if any adverse effect was observed. As a control group, 10 similar patients were evaluated during the same period of 30 minutes after anesthetic induction but prior to vancomycin administration and surgical stimulation. Thirty-one patients (26.72%) developed an adverse effect, mainly hypotension (29 patients, 25%), which was considered severe in 15 patients (12.93%). Seven patients (6.03%) developed a maculopapular erythema that was associated with hypotension (Red-Man's syndrome) in 5 patients and with bronchospasm in 1 patient. The incidence of adverse reactions in children (20%) was similar to the overall incidence. Only 1 patient in the control group (10%) developed hypotension during the period studied. The incidence of adverse reactions was not related to age, body weight, vancomycin dose administered per kilogram body weight, type of surgical procedure, or associated disease. Mean duration of the infusion was similar in patients with and without adverse responses (34.60 +/- 12.41 minutes and 37.38 +/- 14.55 minutes, respectively). It is concluded that perioperative prophylaxis with vancomycin in cardiac surgery produces a high and unpredictable risk of significant hypotension.