Objective: Leptin may be associated with cardiovascular disease. We tested to determine whether leptin is a marker for first-ever acute ischemic stroke (AIS) in a nested case-referent study. Methods: Consecutive patients with first-ever AIS from May 2017 to December 2017 were included. Referents were matched for sex, age and body mass index. Serum leptin levels and routine tests were examined in both groups. Results: The median serum level of leptin in the stroke patients was 14.3 (interquartile range [IQR], 7.2-21.7) ng/ml, which was significantly higher (P < 0.001) than in the referents (10.7; 5.7-13.6 ng/ml). There was a positive correlation between serum level of leptin and National Institute of Health Stroke Scale score (r[Spearman] = 0.43, P < 0.001). In addition, serum leptin levels paralleled lesion size. Median serum level of leptin in patients with small lesions, medium lesions and large lesions was 7.3 (IQR, 5.3-14.3) ng/ml, 13.9 (IQR, 7.0-21.3) ng/ml, 20.5 (IQR, 12.4-32.7) ng/ml, respectively (analysis of variance: P < 0.001). In the univariate model matching for sex and age, leptin as a continuous variable was associated with AIS, after adjustment for possible confounders (odds ratio [OR] 1.07, 95% confidence interval KU: 1.04-1.11; P < 0.001). After adjusting for all other factors, leptin remained an independent stroke predictor with an adjusted OR of 1.03 (95% CI, 1.00-1.10; P = 0.006). Interestingly, the association between AIS and leptin level was more pronounced among men (adjusted OR = 1.05, 95% CI: 1.01-1.12; P < 0.001) when compared with women (adjusted OR = 1.03, 95% CI: 1.10-1.11; P = 0.009). Conclusion: Serum leptin is associated with first-ever AIS, lesion size and stroke severity in a Chinese cohort.