Laser mode partition noise is an important source of noise in optical transmission systems operating with multimode lasers and dispersive fibres, e.g. single-mode fibres with non-zero material dispersion. The letter describes investigations which confirm the importance of considering laser mode partition noise when practical optical systems are projected. It also gives theoretical estimations and measurement results which are in good agreement, and demonstrates the influence of laser mode partition noise on the transmission quality of an analogue optical system.