The validity of drug exposure measurement based on pharmacy records was investigated taking into account completeness of data, drug compliance, and different methods of drug exposure measurement in pharmacy records. Data on prescription drug use were collected from home inventories and community pharmacies in a survey on drug use and compliance in 115 elderly people. To compare drug exposure in pharmacy records with exposure in the home inventory, three different methods for exposure measurement in pharmacy records were used. Two employed a fixed time window of 30 and 90 days, respectively, and the third method was based on the calculated duration of use of a prescription ("legend time"). Drug exposure in the home inventory was taken as the gold standard and sensitivity, specificity, and positive predictive values of the different methods were calculated for the most frequently used drugs and drug categories. The specificity and positive predictive value of all three methods was generally high (0.93-1.00 and 0.67-1.00, respectively). The 90-day fixed method and the legend time method generally showed high sensitivity (range: 0.67-1.00 and 0.63-0.83, respectively) for drugs that were used on a chronic basis, while the 30-day fixed method had poor sensitivity (range: 0.29-0.69). Drugs that were used according to the home inventory but not according to the pharmacy records methods could be almost completely retrieved in the pharmacy records of a one-year period showing that these records were virtually complete with regard to prescription drugs. We conclude that computerized pharmacy records can be a reliable source of the true drug exposure as estimated in a home inventory, when adequate attention is paid to the definition of the exposure time-window and when these records are comprehensive with regard to prescription drugs.