A decrease in mechanical complications after ST-elevation myocardial infarction may have contributed to improved survival rates associated with reperfusion by primary percutaneous coronary intervention (PCI). Mechanical complications occurred in 52 of 5,745 patients (0.91%) in the largest reported randomized trial in which primary PCI was the reperfusion strategy. The frequencies were 0.52% (30) for cardiac free-wall rupture (tamponade), 0.17% (10) for ventricular septal rupture, and 0.26% (15) for papillary muscle rupture (3 patients had 2 complications). Ninety-day survival rates were 37% (11) for cardiac free-wall rupture, 20% (2) for ventricular septal rupture, and 73.3% (11) for papillary muscle rupture. These mechanical complications occurred at a median of 23.5 hours (interquartile range 5.0 to 76.8) after symptom onset and were associated with 44% (23 of 52) survival through 90 days, which accounted for 11% of the 90-day mortality. Factors associated with mechanical complications were older age, female gender, Q waves, presence of radiologic pulmonary edema, and increased prerandomization troponin levels. In conclusion, rates of mechanical complications are lower with primary PCI than those previously reported after fibrinolytic therapy.