The relation between obesity and stroke outcome has been disputed. This study was aimed to determine the association of body mass index (BMI) with mortality and functional outcome in patients with acute ischemic stroke. Data were from a national, multi-centre, prospective, hospital-based register: the ChinaQUEST (Quality Evaluation of Stroke Care and Treatment) study. Of 4782 acute ischemic stroke patients, 282 were underweight (BMI < 18.5 kg/m2), 2306 were normal-weight (BMI 18.5 to < 24 kg/m2), 1677 were overweight (BMI 24 to <28 kg/m2) and 517 were obese (BMI ≥ 28 kg/m2). The risks of death at 12 months and death or high dependency at 3 and 12 months in overweight (HR: 0.97, 95% CI: 0.78-1.20; OR: 0.93, 95% CI: 0.80-1.09; OR: 0.95, 95% CI: 0.81-1.12) and obese patients (HR: 1.07, 95% CI: 0.78-1.48; OR: 0.96, 95% CI: 0.75-1.22; OR: 1.06, 95% CI: 0.83-1.35) did not differ from normal-weight patients significantly after adjusting for baseline characteristics. Underweight patients had significantly increased risks of these three outcomes. In ischemic stroke patients, being overweight or obese was not associated with decreased mortality or better functional recovery but being underweight predicted unfavourable outcomes.