Objective: To determine the relation to cost of different aspects of the management of primary care among group practices within a health maintenance organization network. Measures: A cross-sectional survey study of medical practices conducted with Blue Cross Blue Shield of Minnesota, St Paul. The subjects were group practices accepting financial and administrative responsibility for primary care services in the managed care plans of Blue Cross Blue Shield of Minnesota. One hundred twelve primary care practices and 153 397 enrollees were included in this analysis. The principal resource use measure in this study was nonhospital cost per member per year estimated from payments to providers plus subscriber-eligible liability. Results: The medical directors' responses revealed considerable variability in the management of primary care in these 112 practices. Group practice characteristics consistently associated with lower nonhospital cost were patient identification of a primary care physician, cost of care profiling, more frequent physician profiling, more patients per hour in the clinic, a higher proportion of primary care physicians in the specialty of family or general practice, and a greater number of physicians in the group practice. Conclusions: Results of this study demonstrate substantial variation in the management of primary care among group practices participating in a health maintenance organization network. These differences are associated with significant variation in the nonhospital cost of care for enrollees.