Skeletal muscle changes were evaluated in patients suffering from hemiparesis after stroke. Concentric needle EMG and single fiber EMG of the paretic gastrocnemius medialis muscle were performed. Maximal amplitude of H, T and M responses in calf muscles of both the affected and the unaffected sides were determined by usual electrophysiological techniques. Muscle biopsy of the lateral gastrocnemius muscle of the affected side was performed to determine the distribution of fiber types and fiber sizes. Fibrillation activity and positive sharp waves occurred in paretic muscles in patients with more recent hemiparesis while the duration of motor unit potentials was prolonged in patients with long-lasting disease. The H/M ratio was increased on the paretic side. The percentage of type 1 fibers was augmented in most patients with normal mean diameter and low atrophy factor. The percentage of type 2 fibers was reduced with decreased mean diameters and with a high atrophy factor. Such changes may be related to inactivity or transsynaptic degeneration of type 2 motoneurons as a consequence of the interruption of the corticospinal tract. Increased percentage of type 1 fibers may be due to a collateral reinnervation process or a motor unit type transformation.