The Institute of Medicine reported unacceptably high rates of medical error but did not identify whether this is a growing or a stabilized problem. This study used longitudinal data from all acute care hospitals in Pennsylvania to track rates of acuity-adjusted iatrogenic atelectasis, cardiac complications, complications in general, decubitus ulcers, gangrenous ulcers, falls, hemorrhage, mortality, penumonia, post-procedural infections, treatment complications, and urinary tract infections, from 1994 to 1997. More than one-half of these adverse events increased during this time. It is important to identify the causes and correct trends in hospital systems that may produce an increase in preventable adverse events.