The properties and fiber dependence of distributed Raman pre-amplifiers and, in particular, the effects of Rayleigh backscattering, which limits the sensitivity improvements that can be realized, is discussed. Bit-error-rate (BER) measurements show improvements In the effective receiver sensitivity of 6.2 and 7.0 dB for dispersion shifted fiber (DSF) and silica-core fiber (SCF), respectively. Theoretical predictions of the effective noise figure based on measured fiber parameters indicate improvements of 6.6 dB for DSF and 7.4 dB for SCF in good agreement with the measured optical SNR. The optimum receiver sensitivity is obtained in DSF with less than 600 mW of pump power, whereas the 0.8-dB superior performance of the SCF requires approximately 1 W.