The feasibility of identifying candidates for vaccination by analyzing patients' use of prescribed drugs as recorded in a computerized database was investigated. Pharmacists at a 45-bed hospital serving U.S. Armed Forces in Germany developed a list of drugs used to treat patients with diagnoses that warrant influenza or pneumococcal pneumonia vaccination. In December 1988 and April 1989, the facility's pharmacy database was searched for patients who had received the infection-risk-indicating drugs during September 1, 1988-March 30, 1989. Medical records of patients identified were screened to verify the indications for vaccination. Patients at risk were sent letters and prescriptions written by a pharmacist to inform and remind them of their need for vaccination. Of 109 patients for whom records were screened, 80 (73.4%) had valid indications for immunization. Of these 80 patients, 48 (60%) accepted the invitation to be immunized. The 80 patients in need of vaccination received 151 individual prescriptions; 77 doses (51%) were administered to 48 patients. Screening by pulmonary, cardiovascular, and hypoglycemic drug use identified the largest numbers of patients in need of vaccination, as did age over 64 years. Patients with more than one risk factor for infection were significantly more likely to accept vaccination than patients with only one risk factor. Pharmacoepidemiologic programs can help pharmacists to fulfill their role as advocates of immunization.