In order to improve the performance of multiuser (MU) multiple-input multiple-output (MIMO) systems with imperfect channel information, this paper investigates mode selection between single-user (SU) transmission and MU broadcast communication with block diagonalization. By quantifying the rate loss and approximating the ergodic sum-rate, we present the impact of imperfect channel information on the performance. Results show that, as expected, the MU-MIMO system is interference-limited at high signal-to-noise ratio (SNR). Mode selection is performed based on the theoretical analysis, and we present simulation results to verify the analytical derivations.