This paper presents a new approach to the classification of ambulatory care into isoresource consumption groups. In contrast to classification schemes based on visits, this case-mix approach creates an index based on resources used by diagnostic categories by a patient during a year. An application of this method to a primary care, group practice data base produced resource consumption groups with coefficients of variation in an acceptable range compared with the coefficients of variation of the diagnosis-related groups used to classify inpatient care.