A patient with an infarct limited to the paramedian thalamus and upper mesencephalon on the right side suffered a conjugate upgaze palsy associated with a monocular paresis of downward gaze in the ipsilateral eye (vertical "one-and-a-half" syndrome). This paresis involved tonic and phasic components. Vertical oculocephalic movements and conjugate horizontal gaze were normal. It is suggested that the unilateral lesion destroyed the fibres of the posterior commissure and the descending fibres to the ipsilateral subnucleus of the inferior rectus and contralateral subnucleus of the superior oblique just after they decussate, probably above the level of the third nerve nucleus. A complex disturbance of vertical gaze may be due to a unilateral thalamo-mesencephalic lesion.