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In this paper, we investigate the performance of opportunistic scheduling in uplink cognitive radio (CR) systems by taking into account the CR users' spectrum sensing reliability. In uplink CR systems, each secondary transmitter (CR user) has a spectrum sensing responsibility to protect the primary system, and the secondary receiver (base station) schedules the transmission opportunities for the CR users. We propose novel optimal and suboptimal scheduling schemes by simultaneously taking into account the spectrum sensing reliability and the data channel quality. Analytical performance results for the proposed suboptimal scheduling schemes show that the spectrum sensing reliability should be considered for scheduling to maximize the capacity of the secondary system. Moreover, we also analyze the achievable multiuser diversity (MUD) gain of one of the proposed suboptimal scheduling schemes, which is based on the linear combination of the sensing channel and the data channel qualities. We show that the MUD gain of uplink CR systems grows significantly slower than that of conventional multiuser systems, particularly if the quality of the sensing channel is poor. Analytical and simulation results confirm that the proposed optimal and suboptimal scheduling schemes taking into account the spectrum sensing reliability and the data channel quality yield significant performance gains compared with conventional opportunistic scheduling.