Three patients who had slowly alternating skew deviation are described; each had elements of the Sylvian aqueduct syndrome. This combination of signs supports a pretectal location for lesions associated with alternating skew movements. Postmortem examination of a patient who died of chronic herpes simplex encephalitis showed extensive demyelination and periaqueductal spongiform degeneration; there was preservation of the oculomotor and trochlear nuclei, the medial longitudinal fasciculus, vestibular nuclei, and the interstitial nucleus of Cajal bilaterally. The slowly alternating dysconjugate vertical movements bear a resemblance to both see-saw nystagmus and the ocular tilt response.