Researchers in Ireland studied the prevalence of potentially inappropriate medications (PIMs) using STOPP (Screening Tool of Older Persons' potentially inappropriate Prescriptions) criteria and Beers criteria in 600 consecutive older people hospitalized with acute unselected illnesses. The incidence of significant adverse drug events (ADEs) in this cohort was 26.3%, two-thirds of the ADEs being causal or contributory to admission. Nearly 69% of the ADEs that were causal or contributory to admission were in turn avoidable according to accepted ADE avoidability criteria. Clinically significant ADEs were listed in STOPP criteria 2.54 times more often than in Beers criteria. After adjusting for age, sex, comorbidity, dementia, baseline activities of daily living function, and number of medications, STOPP criteria PIMs were significantly associated with serious, avoidable ADEs that cause hospitalization in older people, whereas Beers criteria PIMs were not.