In this letter, cognitive two-tier femtocell networks are considered, where each femtocell transmits over subchannels that are considered idle by spectrum sensing. Since femtocells transmit on the 'idle' subchannels based on imperfect cognitive sensing, co-tier and cross-tier interference will arise. We derive the average uplink throughput of cognitive femtocell networks, where fractional power control and adaptive modulation are adopted. Besides, by maximizing the system throughput, the optimal channel access probability of each femtocell is obtained. Simulation results verify the validity of the analysis.