In this paper we consider the coexistence issue between OFDM-based WLANs of the 802.11a family and time-hopping ultra wideband (TH-UWB) systems. A general analytical expression is provided for the bit error rate (BER) of the OFDM transceiver operating in the presence of UWB interference and the interferences caused by the multipath propagation phenomenon on the 802.11a OFDM signal, comprising intersymbol interference (ISI) and interchannel interference (ICI). Numerical results are provided to illustrate the severity of the performance degradation as a function of the intensity of the UWB interferers to the 802.11a receiver and their proximity to the latter. The effect of the UWB system parameters to reduce its inflicted interference is also addressed. Such design can allow for an acceptable level of interference for these systems to coexist.