Hemiballismus is a rare movement disorder characterized by involuntary, large amplitude movements of the limbs of 1 side of the body. We describe the case of a man in his late sixties with slurred speech, agitation, and right-sided hemiballismus resulting from a left thalamic hemorrhagic stroke. Treatment with haloperidol was unsuccessful, but both the hemiballismus and agitation diminished significantly after initiation of olanzapine (Zyprexa). The improvement in the hemiballismus was quantified by recording the number of hemiballistic movements that occurred while the patient performed standardized 30-minute sessions (daily for 5d). With the first task (reaching within the base of support while seated), the average number of hemiballismic movements per session decreased from a baseline of 23.5 to 3.0 in the upper extremity and from 20.5 to 7.0 in the lower extremity. With the second task (catching a ball while seated), the abnormal movements decreased from 52 to 6.3 in the upper extremity and from 34.5 to 2.7 in the lower extremity. This case suggests that olanzapine may be a valuable pharmacologic alternative for patients with hemiballismus.