Patients from one municipality in Sweden utilizing primary health care (PHC) during 1998 and 1999 have been categorized into 81 groups. The groups show each patient's own case-mix in terms of illness. Grouping was carried out using the case-mix instrument adjusted clinical groups (ACG), developed by the School of Hygiene and Public Health at Johns Hopkins University in Baltimore, USA. The resulting pattern provided a more adequate reflection of the scope of primary care's task than that yielded by diagnoses alone. Changes over time in terms of illness patterns for a population could be described, analysed and assessed from medical and health economic perspectives. One of the conclusions from this study was that the ACG instrument is a relevant tool in describing the outcome of work by the primary health care centre. The ACG is of interest in the improvement of the quality of primary care in Sweden. The ACG should be a driving force in the development of health indices in both national and international comparisons, as a result of its focus on the health status of patients and populations instead of on diagnoses and diseases.