Summary: We describe a simple technique for the numerical calculation, within any desired degree of accuracy, of the pairwise error probability (PEP) of space-time codes over fading channels. This method applies also to the calculation of \(E[\mathbb Q(\sqrt{\xi})]\) for any nonnegative random variable \(\xi\) whose moment-generating function \(\phi\xi(s)=E[\exp(-s\xi)]\) is known. Its application to the multiple antenna independent Rayleigh-fading channel and to the Rayleigh block fading channel is discussed, and illustrated by two simple examples.