The purpose of this study was to document adverse drug reaction (ADR)-related hospitalizations from a nursing facility population. This 4-year prospective observational study used monthly repeated measures of 332 residents present for 30 or more days. The review included admission and monthly drug regimen review for each resident. Each probable ADR was sent with monthly reports to attending physicians and charge nurses. There were 64 ADR-associated hospitalizations in 52 of the 332 residents (15.7%). The most common events were for nonsteroidal anti-inflammatory drugs (NSAIDs) (30), psychotropic-related fall with fracture (14), digoxin toxicity (5), and insulin hypoglycemia (4). Five patients had recurrence of the hospitalization for the same problem. A significant factor noted between ADR hospitalized and non-ADR residents was the number of medications per patient (7.9 +/- 2.6 vs 3.3 +/- 1.3) for the same number of problems. Adverse drug reaction-related hospitalizations may affect as many as one of every seven nursing home residents and appear to be related to polypharmacy as well as inattention to patient history of contraindications and previous ADRs.