Objective -Determine elderly inpatients' risk for ADRs and characterize the events. Design -This is a post-hoc analysis of a comprehensive inpatient ADR survey. Charts were reviewed every four days on all internal medicine service inpatients (1024 patients over four months). Chart reviews were enhanced by potential indicators such as nurses' and pharmacists' reports; targeted drug orders; 'now', 'stat', and 'hold' orders; off-service physician consults; incident reports; transfers-to-ICU; and abnormal serum drug concentrations. Potential ADRs were classified according to organ system affected, pharmacological type, severity, and Naranjo causality scale. Setting -Internal medicine wards of a 350-bed county general hospital. Results -Of 1024 inpatients, 301 were elderly. Overall, 237 patients had an ADR (23%). Elderly patients accounted for 89 (37.5%) of the 237 patients experiencing an ADR. The ADRs experienced by the elderly tended to be more severe ( p <0.05) and less idiosyncratic ( p <0.05). However, no preferences for organ system ( p > O . l ) or differences in causality rating ( p = 0.25) were detected. When statistically controlled for female gender, renal function and number of drugs, age was no longer a risk factor for ADR occurrence. Conclusions -The elderly experience more ADRs. However, female gender, decline in renal function and polymedicine are the independent factors that account for the elderly's risk. Furthermore, the elderly's ADRs tend to be more severe and an extension of the drug's pharmacology. Therefore, ADR prevention is both important and possible.