Therapy to restore functional movement of stroke patients is based on assumptions about the deficits that occur in motor control as a result of stroke. Success has been limited, perhaps as a result of insufficient information concerning the characteristics of movement after stroke. In this pilot study, the Waterloo Spatial Motion Analysis Recording Technique (WATSMART™), an optoelectric motion analysis system, was used with surface electromyography to measure voluntary reaching in the impaired and unimpaired arms of 5 subjects with left hemiparesis. The ability to reach in a smooth coordinated way was significantly poorer in the impaired arms than in the unimpaired arms, for which scores were essentially normal. The patients were less able to activate the muscles of the impaired arm and, as a result, used a greater percentage of the maximum activity they could generate to complete the unresisted reaching task. The electromyographic differences between arms, however, did not reach significance. The results corroborate previous findings and show that movement deficits of particular patients can be diagnosed precisely with kinematic analysis and electromyography. If greater precision in diagnosis were available clinically, more effective therapy might be developed.