Fifteen patients with internuclear ophthalmoplegia (INO) were examined by simultaneous electro-oculography of each eye. In 11 patients the INO was bilateral and in 4 patients unilateral. In 6 of the bilateral cases, the INO was asymmetric. A classic presentation of the INO was shown by only 2 patients. The rest had additional disorders of eye mobility (INO plus). The high percentage of additional affections, which were not often clinically apparent, consisted of slowing of abduction (45%), impairment of vertical movements (80%) and convergence (30%), and dysmetric overshoot of the abducing eye (65%). Impairment of vertical movements were combined in 3 patients with unilateral INO. Isolated downward vertical gaze paresis was found in 4 patients. Thus an investigation by quantitative methods demonstrates the very rare occurrence of isolated lesions of the medial longitudinal fascicle (MLF). Affections of the MLF mostly are combined with lesions of the surrounding reticular formation and therefore with additional optomotoric defects.