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In this paper, a new algorithm is proposed for cooperative beamforming and power allocation (CBPA) in the downlink of multi-input multi-output (MIMO) cognitive radio systems. Increasing the efficiency of spectrum usage is the main goal of the cognitive radio systems by servicing a number of secondary users in addition to the primary user. The proposed CBPA algorithm is developed based on a criterion in which the total signal to interference plus noise ratio (SINR) of the secondary users is maximized subject to guarantee the primary user's quality of service and a maximum transmit power. In addition, the CBPA algorithm provides the same quality of service for all secondary users. The transmitter and receiver beamforming vectors along with allocated powers to users are estimated by the CBPA algorithm in an iterative manner. The performance of the CBPA algorithm is evaluated by computer simulations and compared with that of the uplink-downlink duality method.