Aims To assess the frequency and cost of drug reactions causing or prolonging hospitalization. Methods All patients admitted to an internal medicine ward over 6 months were evaluated to identify serious adverse reactions. The number of drug classes on admission or at the time of the adverse drug reaction (ADR) was counted. Excess ADR-related hospital stay was computed using a) raw excess duration of hospital stay, b) correction of duration of hospital stay by age, sex, and number of drug classes, and c) estimation by investigator of excess hospital stay. Results Three hundred and twenty-nine patients were evaluated: 212 male, 117 female, mean age 57.2 (males: 52.2, females: 66.2 ( P<0.05)), range 17-95 years.