Background Atrial fibrillation (AF) is a common complication of heart surgery. Previous studies have shown that there is an association between postoperative AF and prolongation of hospital length of stay. No previous trials have been primarily directed at demonstrating that the use of drugs that prevent AF would shorten length of stay and reduce the costs of postoperative care. Methods A randomized, double-blind, placebo-controlled trial of metoprolol was performed in patients immediately after nonemergent heart surgery. Metoprolol was given orally at a dose of 100 mg per day after the patient's arrival in the intensive care unit until hospital discharge or 14 days, whichever was sooner. This dose was increased to 150 mg per day after the enrollment of 411 patients. The primary outcome measure of the study was hospital length of stay from admission to intensive care unit until hospital discharge. There were 1000 patients enrolled, evenly distributed to the metoprolol and placebo groups. Results There was a 20% reduction in the risk of AF developing with metoprolol, from 39% of patients to 31% of patients (P =.01). There was no effect of treatment on hospital length of stay, which was 152 ± 61 hours for placebo and 155 ± 90 hours for metoprolol (P = 0.79). The cost of postoperative care in the 2 treatment groups was similar. Conclusion Prophylactic metoprolol reduces the risk of AF after heart surgery. It does not reduce hospital length of stay. Although it is cost effective for the reduction of AF, it did not reduce the overall cost of care. (Am Heart J 2003;145:226-32.)