AIMS:To describe the development of a new test of bimanual performance for adults following Stroke, the Adult-Assisting Hand Assessment Stroke, and to report the evidence of internal and external validity. METHODS:Scale development included: (i) establishing the test situation; (ii) constructing test items; (iii) evaluating internal construct validity by use of Rasch measurement analysis on 144 assessments of adults with hemiparesis, mean age 53 years (SD11.45); and (iv) investigating external validity by correlation to the Jebsen and Taylor Test of Hand Function and the ABILHAND Stroke. RESULTS:The Adult-Assisting Hand Assessment Stroke scale, scored on 19 items using a four-point rating scale, provided a valid measure of bimanual performance. The rating scale structure, goodness of fit, and principal component analysis demonstrated evidence of a unidimensional construct. The strong reliability and high person separation ratio indicated high probability for the scale to be responsive to change. Correlation to outcomes of the Jebsen and Taylor Test of Hand Function and the ABILHAND Stroke indicated strong external validity. CONCLUSION:Using two hands together is a critical aspect for performance of most daily life tasks. However, assessments of hand function commonly focus on measuring aspects of unimanual function. The Adult-Assisting Hand Assessment Stroke has the potential to contribute new and clinically important knowledge to stroke rehabilitation by providing an observation-based valid functional measure of bimanual performance. Implications for rehabilitation Hand function assessments commonly focus on unimanual aspects, although the use of two hands together is critical to perform most daily life tasks. The Adult-Assisting Hand Assessment Stroke measures how effectively a patient with a hemiparesis uses his/her affected hand together with the unaffected hand to perform bimanual tasks. The Adult-Assisting Hand Assessment Stroke contributes a new and clinically important aspect to stroke rehabilitation by providing a valid bimanual observation-based measure to guide intervention and measure change over time.