Enhanced DCF (EDCF) is currently under review as the new standard for quality of service in IEEE 802.11 wireless LANs. In EDCF, per-flow differentiation is achieved by maintaining separate queues for different traffic categories (TCs). However, due to its static QoS parameter setting, EDCF does not perform adequately under high traffic load. We present an extended performance model of EDCF and possible conditions for network getting overloaded. With this extended model, we show that the overall throughput of a network can be improved by changing the distribution of the number of active stations over a set of TCs. Hence, we propose to dynamically re-allocate flow priorities evenly in order to maintain high system performance while providing QoS guarantee for individual real-time flows. Our scheme has several interesting features: (1) Performance of EDCF is improved; (2) Low priority flows are not starved under high traffic load; (3) Misuse of priority can be easily handled. Simulations are conducted for both infrastructure-based and Ad-hoc models. Results show that dynamic priority re-allocation does not decrease throughput of real-time flows under low to medium loads, while considerable improvement over EDCF is obtained even under high loads, making it easy to support multimedia applications.