In this paper a two-wheeled wheelchair system is presented. The proposed wheelchair moves accordingly to the motion of the user's center of gravity, resulting in an unstable system along the pitch direction. A motion controller based on the synthesized pitch angle disturbance observer (SPADO) is introduced to keep the system in equilibrium. The performance of the entire system is strongly affected by the inertia parameters variation, which must be taken into account in order to design a robust controller. Finally experimental results will be presented in order to prove the theoretical results.