Gait and balance disorders are common in patients with parkinsonian syndromes, but the pathophysiology of these symptoms is still poorly understood. This study examined the initiation of gait in patients with progressive supranuclear palsy (PSP, n=10), characterized by the presence of severe postural instability, and controls (n=43). We used a force plate to measure the step length, and the antero-posterior and vertical velocities of the centre of gravity (CG) during gait initiation, in natural and fast gait conditions. In controls, during the swing limb period, there was a fall in the CG, which was reversed before foot contact. When controls were asked to walk faster, the fall in the CG increased but the ability to brake the fall in CG was unchanged. In PSP patients, length and maximal velocity of the first step were reduced compared to controls and no anticipatory braking in the CG fall occurred prior to the foot-contact, especially in the fast gait condition. The results suggest that normal subjects actively brake the fall in the CG prior to foot-contact. How this phenomenon participates in balance control is unknown. We hypothesize that the absence of active braking of the fall in the CG prior to foot-contact, observed in PSP patients, could contribute to the postural instability, characteristic of this disorder.