Background Concern exists regarding the results of primary angioplasty for acute myocardial infarction when the procedure is performed during night hours. Methods and Results Between June 1994 and January 1997, 491 patients with acute myocardial infarction who underwent primary angioplasty procedures were consecutive registered in the Maximal Individual Therapy in Acute Myocardial Infarction (MITRA) study. Three hundred seventy-eight patients (77%) were treated during the day and 113 (23%) at night. Baseline characteristics showed no major differences between the 2 groups. Prehospital delay time was 60 minutes shorter during the night (median value 180 minutes for day, 120 minutes for night, P = .005), and in-hospital time to treatment was 9 minutes longer (median value 85 minutes day, 94 minutes night, P = .037). Patients treated during the night more often received angiotensin-converting enzyme blockers (61.4% day, 76.1% night, P = .004) and the so-called optimal adjunctive therapy (54% day, 64.6% night, P = .045). There were no differences concerning clinical events between the 2 groups. Hospital mortality was 8.7% during the day and 5.3% during the night (univariate analysis P = .238; logistic regression P = .653). Conclusions In a clinical setting, primary angioplasty for acute myocardial infarction can be performed safely during the night with a clinically insignificant prolongation of in-hospital time to reperfusion compared with practice during the day. (Am Heart J 1999;138:1111-7.)