Objectives: To measure vertical and horizontal responses to optokinetic (OK) s timulation and investigate directional abnormalities of quick phases in progress ive supranuclear palsy (PSP). Methods: Saccades and OK nystagmus were studied in six PSP patients, five with Parkinson’s disease (PD), and 10 controls. The OK stimulus subtended 72°horizontally, 60°vertically, consisted of black and whit e stripes, and moved at 10-50°/s. Results: All PSP patients showed slowed volu ntary vertical saccades and nystagmus quick phases compared with PD or controls. Small, paired, horizontal saccadic intrusions (SWJ) were more frequent and larg er in PSP during fixation. Vertical saccades were transiently faster at the time of SWJ and horizontal saccades in PSP. During vertical OK nystagmus, small quic k phases were often combined with horizontal SWJ in all subjects; in PSP the vec tor was closer to horizontal. Vertical OK slow phase gain was reduced in PSP but , in most PD patients, was similar to normals. The average position of gaze shif ted in the direction of vertical OK stimulus in PSP patients with preserved slow phase responses but impaired quick phases. Conclusions: Vertical OK responses i n PSP show impaired slow phase responses, and quick phases that are slowed and c ombined with SWJ to produce an oblique vector. SWJ facilitate vertical saccades and quick phases in PSP, but it is unclear whether this is an adaptive process o r a result of the disease. A large OK stimulus is useful to induce responses tha t can be quantitatively analysed in patients with limited voluntary range of ver tical gaze. Objectives: To measure vertical and horizontal responses to optokinetic (OK) s timulation and investigate directional abnormalities of quick phases in progress ive supranuclear palsy (PSP). Methods: Saccades and OK nystagmus were studied in six PSP patients, five with Parkinson’s disease (PD ), and 10 controls. The OK stimulus subtended 72 ° horizontally, 60 ° vertically, consisted of black and whit e stripes, and moved at 10-50 ° / s. Results: All PSP patients showed slowed volu ntary vertical saccades and nystagmus quick phases compared with PD or controls. Small, paired, horizontal saccadic intrusions (SWJ) were more frequent and larg er in PSP during fixation. Vertical saccades were transiently faster at the time of SWJ and horizontal saccades in PSP. During vertical OK nystagmus, small quic k phases were often combined with horizontal swj in all subjects; in PSP the vec tor was closer to horizontal. The average position of gaze shif ted in the direction of vertical OK stimulus in PSP patients with preserved slow phase responses but impaired quick phases. Conclusions: Vertical OK responses in PSP show impaired slow phase responses, and quick phases that are slowed and A large OK stimulus is useful to induce responses tha t can be quantitatively analysed in patients with limited voluntary range of ver tical gaze.