A new class of receivers based on widely linear data processing has been recently proposed for data detection in communication systems affected by improper complex noise. In this paper, it is shown that this detection strategy may be applied to wireless communication systems employing multiple transmit and receive antennas and adopting a noncircular modulation. Improved versions of the linear decorrelating and minimum mean square error (mmse) receivers, and of the nonlinear ing and cancellation (V-BLAST) receiver are, thus, developed and analyzed. In particular, we show that the improved receivers outperform the conventional ones both in terms of the error probability and of the capacity to cope with the power disparities that the fading channel may induce on the data streams transmitted by different antennas. Moreover, the improved receivers exhibit satisfactory performance also in systems with a number of transmit antennas exceeding the number of receive antennas. Finally, we also consider the situation in which the propagation channel is not perfectly known to the receiver, and show that the performance of the improved receivers is less sensitive to the channel estimation errors than the conventional receivers.