A survey of drug-related admissions of patients aged 50 years and older was conducted at the Health Sciences Centre, Winnipeg to determine the interrelationship of risk factors, and isolate the effect of age. All nonelective medical admissions were prospectively assessed to determine the role of drug therapy as a contributory factor. Of the 863 eligible admissions, 162 exhibited at least one drug-related adverse patient event (DRAPE) at the time of hospitalization. This accounted for 19% of the admissions (23% of 718 admissions that involved prescription drugs). Although adverse drug reactions were responsible for many DRAPEs (48%), intentional noncompliance (27%), treatment failure (19%), alcohol (14%), and medication error (10%) were also frequent contributing causes. Drugs commonly implicated in DRAPEs were systemic steroids, digoxin, nonsteroidal anti-inflammatory agents, alpha-methyldopa, calcium channel blockers, beta-blockers, theophylline, furosemide, sympathomimetics, thiazides, and benzodiazepines. The risk of a DRAPE was related to the number of diseases prior to admission (r = 0.81; P less than .026) and the number of drugs used (r = 0.77; P less than .001). Age was not correlated with the risk of a DRAPE. Females had significantly more adverse drug reactions, although sex was not a predictor for overall DRAPE risk.