Although medical injuries are recognized as a major hazard in the health care system, little is known about their impact. To assess excess length of stay, charges, and deaths attributable to medical injuries during hospitalization. The Agency for Healthcare Research and Quality (AHRQ) Patient Safety Indicators (PSIs) were used to identify medical injuries in 7.45 million hospital discharge abstracts from 994 acute-care hospitals across 28 states in 2000 in the AHRQ Healthcare Cost and Utilization Project Nationwide Inpatient Sample database. Length of stay, charges, and mortality that were recorded in hospital discharge abstracts and were attributable to medical injuries according to 18 PSIs. Excess length of stay attributable to medical injuries ranged from 0 days for injury to a neonate to 10.89 days for postoperative sepsis, excess charges ranged from 0 dollar for obstetric trauma (without vaginal instrumentation) to 57 727 dollars for postoperative sepsis, and excess mortality ranged from 0% for obstetric trauma to 21.96% for postoperative sepsis (P<.001). Following postoperative sepsis, the second most serious event was postoperative wound dehiscence, with 9.42 extra days in the hospital, 40 323 dollars in excess charges, and 9.63% attributable mortality. Infection due to medical care was associated with 9.58 extra days, 38 656 dollars in excess charges, and 4.31% attributable mortality. Some injuries incurred during hospitalization pose a significant threat to patients and costs to society, but the impact of such injury is highly variable.