The study of 826 geriatric nursing home patients was undertaken by a pharmacist to determine the incidence of adverse reactions to drugs. A formalized adverse drug reaction reporting program was used as the basis for the detection of unwanted drug effects. Approximately 27% of patients experienced a moderate to severe adverse effect. Five groups of drugs accounted for three-quarters of the reactions detected. The majority of these reactions were predictable and preventable. Females over the age of 70 on large quantities of drugs were significantly more susceptible to experiencing an adverse reaction. Physicians implemented 76% of pharmacist initiated recommendations for the treatment of adverse effects and were appreciative of increased pharmacy involvement. The implementation of such recommendations was felt to have substantially reduced the incidence of drug-induced acute hospitalizations. Although the information was generated in nursing homes, it may be generalized to other geriatric specialty areas such as medical wards and extended care units. Several recommendations are provided on how other pharmacists can implement similar clinical services in their areas of professional practice.