In this paper, we evaluate the effect of multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) interference in fading narrowband (NB) channels. We derive the exact bit error rate (BER) of a binary phase-shift keying (BPSK) NB receiver impaired by fading, MB-OFDM UWB interference, and additive white Gaussian noise. We show that for NB signal bandwidths much smaller and much larger than the OFDM sub-carrier spacing a Gaussian approximation (GA) and an impulsive GA (IGA) of the MB-OFDM UWB interference lead to accurate performance predictions, respectively. While for general fading distributions and most NB signal bandwidths of practical interest the exact BER analysis has to be used to obtain meaningful results, both the GA and the IGA are very accurate for Rayleigh fading and arbitrary NB signal bandwidths