This cohort study, conducted within a large health management organization (HMO), examined the frequency of adverse drug events (ADEs) that were attributable to errors elderly patients made in handling their medications. Nearly one-quarter of ADEs were caused by patient error, most often due to failure to administer the medication at the proper dose or failure to follow the clinician's instructions on medication use. The classes of medications most frequently associated with errors (hypoglycemics, anticoagulants, and cardiovascular medications) were similar to those found in prior research. A previous commentary discussed the role of preventable and non-preventable patient errors in contributing to adverse clinical outcomes.