OBJECTIVE: To analyze the use of all subsidized prescription drugs with special attention to the elderly (> or = 70 years of age), including their use of drug combination generally accepted as carrying a risk of severe interactions. DESIGN: Descriptive prevalence study. SETTING: Odense Pharmacoepidemiological Database, Denmark. The fourth quarter of 1994. SUBJECTS: All inhabitants in the county of Funen, Denmark. MAIN OUTCOME MEASURES: Prevalence of drug use for persons < 70 and > or = 70 years of age including number of drugs used and of drug combinations. Occurrence of 45 drug combinations generally accepted as carrying a risk of severe interactions. RESULTS: Among persons less than 70 years, 67.9% used none, 16.5% used one drug and 15.6% used two or more prescription drugs. The corresponding prevalences for the elderly were 35.7%, 15.9% and 48.4%. The 26,337 elderly patients with at least two drugs used 21,293 different combinations. Of the elderly patients who had purchased > or = two drugs, 4.4% had combinations of drugs carrying a risk of severe interactions. CONCLUSIONS: Most elderly use drugs and usually several drugs concomitantly. The elderly form a heterogeneous group of drug users. Drug combinations carrying a risk of severe interactions are relatively frequent.