Background and Purpose: Cardiovascular risk factors, which are overall more prevalent in men, are considered the major risk factors for strokes among young adults. However, recent European data found the incidence of strokes to be higher in young women. Using a large US claims sample, we examined sex differences in the index stroke rate of young adults. Methods: We performed a retrospective cohort study of enrollees in a 10% random sample of PharMetrics, a nationally representative claims database of insured Americans from 2001 to 2014. Outcomes were index ischemic stroke events, based on inpatient admissions using International Classification of Diseases-Ninth Revision codes. The index stroke rate was estimated from Poisson rate models with time varying covariates for 2-year periods, stratified by sex and age groups. Results: We identified 20 554 index strokes (50.4% women; mean age 63) including 5198 in young adults ages 15 to 54. There was no difference by sex in the index stroke rate in the extremes of age groups 15 to 24 and ≥75 years old. However, in the 25 to 34 and 35 to 44 year age groups, more women had strokes than men (incidence rate ratio: men:women, 0.70 [95% CI, 0.57–0.86]; 0.87 [95% CI, 0.78–0.98], respectively). In contrast, in the 45 to 54, 55 to 64, and 65 to 74 year age groups, more men had strokes (incidence rate ratio, 1.25 [95% CI, 1.16–1.33]; 1.41 [95% CI, 1.18–1.34]; 1.18 [95% CI, 1.12–125], respectively). Conclusions: More young women than men have strokes, suggesting possible importance of sex-mediated etiologies of stroke. Understanding these drivers is critical to stroke treatment and prevention efforts in young adults.