Objective Although the body mass index (BMI) is considered a meaningful parameter for evaluating obesity, the association between the BMI and acute non-cardioembolic stroke remains unclear. We investigated how the BMI was related to patients' background, type of infarction, and infarction location in patients with non-cardioembolic stroke using an acute dual study (ADS) cohort. Methods The ADS trial was conducted between May 2011 and June 2017 in Japan. The BMI classifications were those proposed by the World Health Organization classification: underweight, <18.5 kg/m2; normal weight, 18.5-24.9 kg/m2; overweight, 25-29.9 kg/m2; and obese, ≥30 kg/m2. Results Data from 1,136 patients were analyzed. The median BMI was 23.6 kg/m2 (interquartile range: 21.6-25.8 kg/m2), with a BMI ≥30 kg/m2 in 63 patients (6%), 25-29.9 kg/m2 in 321 (28%), 18.5-24.9 kg/m2 in 692 (61%), and <18.5 kg/m2 in 60 (5%). The group with a BMI ≥30 kg/m2 was the youngest, and the group with a BMI <18.5 kg/m2 was the oldest (p<0.001). The proportion of patients with a history of hypertension (p<0.001), diabetes (p<0.001), dyslipidemia (p<0.001), and statin therapy (p=0.005) increased with increasing BMI. Pontine infarcts were frequent in the following order: obese, overweight, normal weight, and underweight (24%, 18%, 14%, and 13%, respectively; p=0.034). In contrast, cortical infarct were frequent in the order of underweight, normal weight, overweight, and obese at 20%, 19%, 14%, and 3%, respectively (p=0.007). Conclusion Acute stroke patients with a high BMI have more atherosclerosis-related factors in their backgrounds than those with lower BMIs. In addition, the BMI may be a determinant of infarct location in patients with acute stroke.