This paper evaluates the bit error rate performance of a ultra-wideband (UWB) communication system under the impact of both multiple access and narrowband interferences operating in additive white Gaussian noise (AWGN) channels. The narrowband interference (NBI) signal is modeled as the IEEE802.11a signal, which can be approximated by the sum of independent asynchronous tone interferers with arbitrary frequencies. Multiple access interference is assumed a zero mean Gaussian process, where it has been investigated in both perfect and imperfect power control scenarios. It will be shown that the performance of a UWB system is severely deteriorated due to the presence of both types of interference, yet the NBI has more impact on the performance of UWB communication systems compared to the multiple access one.