Pupil sizes were assessed in 100 patients admitted to an intensive care stroke unit over a 1-year period to determine if differences existed and if anisocoria might help in diagnosis. From life-size photographs the pupil diameters were measured accurately, and clinical information compiled by the neurology staff was studied. Of the 19 patients with anisocoria 13 had a final diagnosis of stroke. This incidence of anisocoria, 16%, is similar to that in a normal population (20%). The side of the mydriasis was unrelated to the site of the lesions except when the stroke was due to a brainstem infarction; then ipsilateral Horner's syndrome occurred.