OBJECTIVE: To examine the patterns of use of ambulatory care in Quebec in 1991, with special emphasis on patients who received care from more than 20 physicians. DESIGN: Retrospective study of population-based data. SETTING: Province of Quebec. PARTICIPANTS: All 7,154,591 people eligible for coverage under the Régie de l'assurance-maladie du Québec (RAMQ) (Quebec Health Insurance Plan) in 1991, including 3639 people who received ambulatory care from more than 20 physicians. DATA EXTRACTION: The databanks of the RAMQ. OUTCOME MEASURES: Mean number (and standard deviation) of physician visits and services received, place of visit (clinic or private office), mean cost of services, patient's age, diagnosis, type of specialist visited and social assistance status of frequent users. RESULTS: The patients who obtained ambulatory care from more than 20 physicians received 10 times more medical services than the overall patient population (59.6 v. 5.8), and the mean cost per patient for ambulatory care was also 10 times higher ($1379 v. $136). Almost all of the frequent users visited at least one outpatient clinic, as compared with 37.3% of the overall population. A higher proportion of the frequent users than of the overall population obtained care from specialists (98.9% v. 54.7%), mainly general surgeons and psychiatrists. The most frequent diagnoses among the frequent users were anxiety (36.0%), abdominal pain (24.3%), drug or alcohol dependence (22.2%) and depression (16.4%). CONCLUSIONS: A small proportion of the population obtained ambulatory care from a high number of physicians during the year, leading to high expenses. Identifying and understanding this type of frequent user may be useful in developing strategies to promote more effective health-care-seeking behaviours and reduce overuse.