BACKGROUND: Dual-task capacity, which might be impaired poststroke, is needed for daily functions. Therefore, dual-task capacity should be assessed during rehabilitation. The Dual Overload Interference Test (DO-IT) is a new upper extremity (UE) protocol for assessment, combining The Box and Block Test with the Counting Backwards Test. OBJECTIVES: To validate DO-IT by comparing between (1) young and older healthy, (2) stroke and healthy participants. Additionally, to correlate DO-IT with (3) walking-cognitive dual-task assessment (healthy), and (4) standardized cognitive and EF assessments (stroke). METHODS: A cross-sectional study included younger and older community-dwelling healthy individuals (N = 32), and younger and older individuals with stroke (N = 83). DO-IT was administered to all participants. The #blocks transferred (motor) and #correct numbers counted (cognitive) were recorded for single and dual conditions. The walking-cognitive dual-task test was administered to the healthy participants. Motor and cognitive costs were calculated as the difference between single and dual tasks. The Montreal Cognitive Assessment test (MoCA) and the Color Trail Test (CTT) assessed cognition post stroke. RESULTS: Older healthy adults had significantly lower dual-task motor capacity compared to younger adults (median [interquartile range] blocks: older 26 [21-38], younger 46 [38-52], CONCLUSIONS: UE-Cognitive dual-task capacity is affected post-stroke. DO-IT shows potential for use in stroke rehabilitation and its validity should be further researched.