This study provides national estimates of outpatient prescription drug use based on pharmaceutical marketing data. An analysis of total outpatient exposure from 1971 through 1981, generated from data on population and prescription size as well as total prescriptions, indicated that overall drug use is increasing. Overall use was also analyzed by patient age and sex, geographic region, and physician specialty. Data are presented for the most frequently used drug categories and therapeutic classes. Ranking drug chemicals across different products and therapeutic indications, which is a more precise measure of population exposure, indicated that nearly one of every ten prescriptions in 1981 contained either hydrochlorothiazide or codeine. (JAMA 1984;251:1293-1297)