The authors report a retrospective case-control study of adverse drug events (ADEs) at the LDS Hospital in Salt Lake City from 1990 through 1993. ADEs were detected both by voluntary incident reporting and in an automated fashion using a screening algorithm in the hospital information system followed by expert manual review. Patient cases with an ADE were matched with non-ADE controls from the same period, although 647 of the 2227 ADE cases could not be matched with a control and were excluded from analysis. The authors found a 1.74-day attributable increased length of stay and a $2014 (in 1997) attributable increase in mean cost of stay in the ADE group. The authors calculate a total attributable direct cost of ADEs of almost $4.5 million during the 3-year period, and extrapolate a $1.56 billion national cost annually. The authors review the potential generalizability of their automated detection techniques and the potential for computer-aided decision support to prevent ADEs at their hospital and elsewhere.