The authors report two cases of hemichorea-hemiballism revealing a toxoplasmic abscess in the subthalamic nucleus during AIDS. Despite the great frequency of this opportunistic infection such cases are exceptional. This rarity seems to be explained by the frequent association of other lesions of the basal ganglia which may prevent the movement disorders. Nevertheless an hemichorea-hemiballism in young people must suggest a cerebral toxoplasmosis. Finally a symptomatic treatment is often necessary; in these cases the use of sodium valproate must be considered, this drug being able to induce a clinical improvement.