The article presents the findings from a study of 27 large, multi-specialty medical groups that suggests a new middle range theory that the governance of medical groups is evolving away from professional autonomy due to environmental changes resulting from the expansion of managed care. The article takes a contingency theory approach to the governance of medical groups to determine the locus-of-control for strategic and management decision making. The findings support the contention that successful medical groups limit decision making at the highest level in the organization to a small number of physician leaders, thereby creating a new organizational form, professional oligarchy.