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An opportunistic feedback protocol with multiple thresholds and multiple access probabilities is proposed for multiuser diversity in multiple-input-multiple-output (MIMO) broadcast channels. Users compete to report the channel state information (CSI) via contention-based feedback channels if their scheduling metric exceeds a threshold. The users are divided into two classes according to their scheduling metric. For each class, the users have different random access probabilities and different chances to access feedback minislots. The performance of the proposed feedback protocol is evaluated in a MIMO system with zero-forcing (ZF) receivers in terms of the sum-rate capacity, the feedback load, and fairness. The simulation results show that the proposed feedback protocol significantly reduces the feedback load without sacrificing the sum-rate capacity too much. The proposed feedback protocol can be operated with existing scheduling algorithms without modification.