Specialization of function in the left hemisphere of man, as compared with the right, was studied with special reference to the nature of the impairments seen after left-hemisphere damage. Patients with unilateral lesions of the left or the right hemisphere were compared in the acquisition and subsequent performance of a manual skill requiring several hand movements. Patients with left-hemisphere damage were further subdivided into aphasic and non-aphasic groups. Patients with left-hemisphere damage were further subdivided into aphasic and non-aphasic groups. Patients with left-hemisphere damage, whether aphasic or not, were impaired in the acquisition of the task, relative to patients with right hemisphere damage, although aphasics were most severely impaired. Analysis of the errors made during acquisition indicated that perseverative errors and unique errors (unrelated movements) differentiated the left and right groups, but that sequencing errors did not. It was concluded that a major function of the left hemisphere is the control of changes in limb or articulatory posture, and that its complex verbal and praxic functions are derived from such control.