The increasing use of health information technology, particularly computerized provider order entry (CPOE), has yielded safety benefits but has also been fraught with implementation difficulties. Concern has therefore arisen that the real-world performance of commercial CPOE systems may not match performance seen in research settings or with homegrown systems. This study of CPOE implementation in 5 Massachusetts community hospitals should partly allay those concerns, as preventable adverse drug events declined by nearly one-third over the 5-year study period. However, drug-related near misses increased significantly after CPOE implementation, highlighting the potential for unintended consequences with this technology.