Seven patients with "locked-in" syndrome were studied by prolonged polygraphic recordings. Severe alterations in the sleep pattern were observed in five patients who had bilateral extensive pontine lesions resulting in tetraplegia, facial and pseudobulbar paralysis, and absence of conjugate horizontal gaze. Rapid eye movement (REM) sleep was entirely absent while non-rapid eye movement (NREM) sleep was absent, reduced, or altered. The remaining two patients, both of whom had relative sparing of horizontal gaze and apparently either no or minimal pontine tegmental involvement, showed both REM and NREM sleep with only a minimal alteration in the sleep pattern. The study suggests that, in human beings as in animals, pontine structures near the midline are essential for control of sleep states.