Increasing attention is being paid to inappropriate medication use in nursing homes. However, criteria defining the appropriate or inappropriate use of medication in this setting are not readily available and are not uniform. We used a two-round survey, based on Delphi methods, with 13 nationally recognized experts to reach consensus on explicit criteria defining the inappropriate use of medications in a nursing home population. The criteria were designed to use pharmacy data with minimal additional clinical data so that they could be applied to chart review or computerized data sets. The 30 factors agreed on by this method identify inappropriate use of such commonly used categories of medications as sedative-hypnotics, antidepressants, antipsychotics, antihypertensives, nonsteroidal anti-inflammatory agents, oral hypoglycemics, analgesics, dementia treatments, platelet inhibitors, histamine2 blockers, antibiotics, decongestants, iron supplements, muscle relaxants, gastrointestinal antispasmodics, and antiemetics. These criteria may be useful for quality assurance review, health services research, and clinical practice guidelines. The method used to establish these criteria can be used to update and expand the guidelines in the future.