We consider a MIMO Ricean fading channel with perfect side information at the receiver. We derive an analytic upper bound on the difference between the capacity of this channel and the mutual information that is induced by an isotropic circularly-symmetric Gaussian input. This bound is based on a dual expression for mutual information. If the number of receiver antennas m is at least equal to the number of transmitter antennas n, i.e., m ≥ n, this bound tends to zero as the signal-to-noise ratio tends to infinity. This shows that for this case a uniform power allocation is asymptotically optimal. If m < n such a uniform power allocation need not be asymptotically optimal.