To determine the extent to which adverse drug reactions (ADRs) in elderly patients admitted to hospital are due to inappropriate prescribing, we examined 416 successive admissions of elderly patients to a teaching hospital. Interacting drug combinations and drugs with relative contra-indications (CIs) were common, but not as important in producing ADRs as drugs with absolute CIs or unnecessary drugs. Forty-eight patients (11.5% of admissions) were taking a total of 51 drugs with absolute CIs (3.8% of prescriptions). One hundred and seventy-five drugs were discontinued on or shortly after admission in 113 (27%) patients because they were deemed to be unnecessary. One hundred and three patients (27.0% of those on medication) experienced 151 ADRs of which 75 (49.7%) were due to drugs with absolute CIs and/or that were unnecessary, a significantly higher rate of ADRs (p less than 0.001) than observed for all prescriptions. Of 26 (6.3%) admissions attributed to ADRs, 13 (50%) were due to inappropriate prescriptions. The admission rate per prescription was significantly higher (p less than 0.001) for inappropriate than for appropriate drugs. We conclude that much drug-related morbidity in the elderly population may be avoidable, as it is due to inappropriate prescribing.