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Subcortical hyperintensities (SHs) on brain magnetic resonance imaging are associated with cognitive and gait impairment in elderly but their impact on dual-tasking (performing cognitive tasks while walking) in patients with Alzheimer disease (AD) is unknown. This study explored the costs of dual-tasking in relation to SH severity in AD and normal controls (NCs). Cadence while walking on a treadmill, and speed-accuracy-tradeoff (SAT), on 3 working memory tasks, were measured during single-task and dual-task conditions. Dual-task costs (DTC) on SAT, cadence, and overall DTC were measured for each of these tasks. On visual rating of SH severity, AD and NC groups were subdivided into high-SH and low-SH subgroups. Compared with the NC, the AD group performed poorly on all working memory tasks across both conditions, decreased cadence on dual-tasking, and showed a decrement in overall DTC (all P<0.01). When grouped according to SH severity, the low-SH-NC group performed superiorly on working memory tasks (P<0.001) and the high-SH-AD group (P=0.001) showed a decrease in dual-task costs of cadence. Although the AD group showed a decrement in overall DTC (P<0.01) compared with NC, when assessed in terms of SH severity, the high-SH-AD group showed the largest decrement in DTC (P<0.01). Greater SH severity is associated with a decrement in overall dual-tasking ability in AD.