Non-steroidal anti-inflammatory drugs represent an important drug class in ambulatory care. A utilization study among half a million persons showed that 8.6% could be identified as having used one or more non-steroidal anti-inflammatory drugs (excluding salicylates) in 1987. Data were drawn from a representative sample of pharmacy records which comprise full medication histories of individual patients. Overall utilization of non-steroidal anti-inflammatory drugs was 10.8 defined daily doses/(1000 persons.day). Approximately three quarters of the patients are 'incidental' users and receive non-steroidal anti-inflammatory drugs for a relatively short time (30 days or less). Patients who were identified as 'regular' (31-210 days of therapy) and 'heavy' (greater than 210 days of therapy) users, accounted for 21.2% respectively 4.8% of all users. 'Heavy' users are responsible for 17.3% of all non-steroidal anti-inflammatory drug prescriptions. Especially the elderly and females are prone to be 'heavy' users. Five drugs account for 90.4% of all prescriptions (diclofenac, ibuprofen, naproxen, piroxicam, indomethacin). A total of 71.1% of the patients with more than one prescription for non-steroidal anti-inflammatory drugs switched in therapy. There are two classes of concomitant drug use especially relevant with respect to detecting non-steroidal anti-inflammatory drugs-associated risks: H2 blockers and antacids (belonging to anatomical therapeutic and chemical anatomic class A) and diuretics (belonging to anatomical therapeutical chemical anatomic class C). More than half of the 'heavy' users showed concomitant use of drugs in these classes.