PURPOSE: To present and interpret drug prescription patterns, related to various groups of the population in a Swedish county, in order to estimate the prevalence of drug use in different age groups. METHODS: Data on prescriptions, dispensed March-May 1988-2002, were combined with population statistics of Halland, a county in the south of Sweden, and analysed. Number of defined daily doses (DDD) per 100 inhabitants and day and prescriptions per 100 inhabitants and 3 months were used as indicators of drug prevalence. RESULTS: The total drug exposure in the population of Halland nearly doubled during the 15-year period. The most frequently used drugs overall, in 2002, were psycholeptics (N05), analgesics (N02), antibacterials (J01) and sex hormones (G03). Nearly 30% of the women of 15-69 years were exposed to sex hormones. Multiplied drug prevalence among people above 60 was found for antithrombotic drugs (B01), agents acting on the renin-angiotensin system (C09), sex hormones (G03), serum lipid reducing agents (C10), antidepressants (N06) and drugs for peptic ulcer and GORD (A02B). CONCLUSIONS: The increase in drug prescribing over the 15 years concerned both symptom-related treatments, like hormone replacement therapy, analgesics, antidepressants and drugs for acid-related disorders, as well as preventive treatments, like antithrombotics, lipid-lowering drugs and antihypertensives. The unit DDD/100 inhabitants and day gives a fairly correct measure of the percentage treated for chronic disorders. However, for short-term treatment courses and especially for drug use in children, number of prescriptions/100 inhabitants and adequate period of time, is easier to interpret.