The aim of our study was to evaluate both the incidence and the pathologic and clinical features of extrapyramidal disorders in a population of acquired immune deficiency syndrome (AIDS) patients with cerebral toxoplasmosis. Of 240 AIDS patients evaluated in the 1985-1994 period, 50 of them were diagnosed to have cerebral toxoplasmosis on the basis of the following criteria: occurrence of specific antibodies, computed tomography and/or magnetic resonance imaging (MRI), and regression of the symptoms after specific therapy. Three of 50 (6%) had hemichoreoathetosis. In the first case, the disorder began as a dyskinesia of the left hand that subsequently spread to the whole ipsilateral arm and assumed the features of choreic athetotic movements. The other two cases were characterized by left hemisomatic distal choreic movements. Therapy with pyrimethamine and sulfadiazine led to a complete recovery of the extrapyramidal signs in two cases and to improvement in the third. According to our observations, the onset of these movement disorders could not be related to the dimension of the lesion or to the edema, but to a specific localization in subthalamic nucleus, in subthalamic/pallidal, and pallidal/thalamic pathways. MRI seems the elective tool to perform a more accurate study of the anatomic areas involved in this pathway and to verify their integrity. Cerebral toxoplasmosis in AIDS can be considered as a new etiopathogenic cause of choreoathetosis.