Management of pharmacotherapy by the pharmacist and the general practitioner can be a difficult task in elderly patients in whom there is a high concomitant, long-term drug use. Adequate information on drug use is essential in managing pharmacotherapy as well as in an accurate assessment of drug exposure in pharmacoepidemiologic studies. In this study data from computerized pharmacy records, general-practitioner registries and home interviews with 100 elderly patients were compared. Pharmacy records contained 80% of all the prescriptions found at the home interviews, while in general-practitioner data 40% could be traced. Use of drugs dispensed long ago reduced the validity of pharmacy and general-practitioner data. Data on analgesics (70% was found) and respiratory drugs (68%) were less traceable compared to cardiovascular (83%) and psychotropic drugs (81%). Automated pharmacy records are an important source of longitudinal data on drug use and will improve the assessment of drug exposure in pharmacoepidemiologic studies and optimize pharmaceutical care.