After unilateral cerebral hemisphere stroke, resulting in contralateral arm symptoms but largely sparing higher cerebral function, ipsilateral arm function is generally considered to be unaffected In this study, 8 subjects with acute unilateral cerebral infarction (confirmed by CT scan) and primarily motor deficits underwent 11 computerized and 6 clinical assessments between 11 days and 12 months poststroke, and were compared with 12 normal subjects. Computerized tests comprised 3 pursuit tracking tasks (preview-random, step and a combination of these), designed to measure different aspects of integrated sensory-motor (S-M) function, and 12 tasks aimed at breaking tracking into various sensory, perceptual and motor components (joint movement sense, visual resolution, object perception, static and dynamic visuospatial perception, range of movement, grip and arm strength, reaction time, speed, static and dynamic steadiness) The asymptomatic arm was impaired on all but one of the computerized tests throughout the 12-month period, although to a lesser degree than the symptomatic arm. Grip strength was marginally impaired initially. Incomplete neurological recovery was seen in the asymptomatic arm for all functions except strength, speed and steadiness, possibly indicating their resistance to improvement Clinical assessment detected no asymptomatic arm impairment and only a mild transient deficit of higher mental function. Our data suggest that (1) all cerebral hemisphere areas involved in S-M functions can exert some degree of bilateral motor control; (2) ipsilateral influence is never greater than contralateral influence, and is usually considerably less; and (3) the proportion of ipsilateral to contralateral control is closely related to the degree of continuous sensory feedback required by the particular task The mechanism and degree of ipsilateral dysfunction can be explained by a 3-tier cerebral model of S-M integration comprising a lower level of functions with high contralateral specificity (somatosensory and motor), a middle level of non-limb-specific partially lateralized functions (ideomotor praxis and visuospatial perception) and an upper level of global mental activities (intellect, alertness, etc.).