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We use a session-based workload to measure the performance of a commercial Web server. We define a session as a sequence of client requests. An overloaded Web server can experience a severe loss of throughput when measured as the number of completed sessions. Moreover, the overloaded Web server discriminates against longer sessions. Session based admission control (SBAC), introduced in (Cherkasova and Phaal, 1998; 1999), prevents a Web server from becoming overloaded and ensures that longer sessions can be completed. We propose two new adaptive admission control strategies: hybrid and predictive, aiming to optimize the performance of the SBAC mechanism. The hybrid strategy is based on a self-tunable admission control function, which adjusts itself accordingly to variations in traffic loads. It shows improved performance results for workloads with medium to long average session length. The predictive strategy estimates and predicts the number of new sessions it can accept and still guarantee processing of all future session requests. It consistently shows the best performance results for different workloads and different traffic loads.