Rigorous modeling of noise and intersymbol signal interference in optical wavelength division multiplexing receivers with arbitrary electrical filtering was presented. Simulations of a WDM back-to-back system was performed with 32 signal channels. Bit-error-rate was evaluated as a function of received power at the input to the pre-amplifier. Results indicated that for a filter with a narrow bandwidth, the impulse response was spread out, and the detected bit was affected by the impulse response from previous bits.