In this paper, throughput of cognitive multiple-input multiple-output (MIMO) systems operating under quality-of-service (QoS) constraints is studied. It is assumed that transmission power and the covariance of the input signal vector are varied depending on the sensed activities of primary users in the system. Considering the reliability of the transmission and channel sensing results, a state-transition model is provided. Effective capacity is determined, and expressions for the first and second derivatives of the effective capacity are obtained at SNR = 0. The minimum bit energy requirements in the presence of QoS limitations are identified. Numerical results are provided.