BACKGROUND: Despite women having fewer traditional risk factors (eg, hypertension, diabetes), strokes are more common in women than men aged ≤45 years. This study examined the contributions of traditional and nontraditional risk factors (eg, migraine, thrombophilia) in the development of strokes among young adults. METHODS: This retrospective case-control study used Colorado’s All Payer Claims Database (2012–2019). We identified index stroke events in young adults (aged 18–55 years), matched 1:3 to stroke-free controls, by (1) sex, (2) age±2 years, (3) insurance type, and (4) prestroke period. All traditional and nontraditional risk factors were identified from enrollment until a stroke or proxy-stroke date (defined as the prestroke period). Conditional logistic regression models stratified by sex and age group first assessed the association of stroke with counts of risk factors by type and then computed their individual and aggregated population attributable risks. RESULTS: We included 2618 cases (52% women; 73.3% ischemic strokes) and 7827 controls. Each additional traditional and nontraditional risk factors were associated with an increased risk of stroke in all sex and age groups. In adults aged 18 to 34 years, more strokes were associated with nontraditional (population attributable risk: 31.4% men and 42.7% women) than traditional risk factors (25.3% men and 33.3% women). The contribution of nontraditional risk factors declined with age (19.4% men and 27.9% women aged 45–55 years). The contribution of traditional risk factors peaked among patients aged 35 to 44 years (32.8% men and 39.7% women). Hypertension was the most important traditional risk factor and increased in contribution with age (population attributable risk: 27.8% men and 26.7% women aged 45 to 55 years). Migraine was the most important nontraditional risk factor and decreased in contribution with age (population attributable risk: 20.1% men and 34.5% women aged 18–35 years). CONCLUSIONS: Nontraditional risk factors were as important as traditional risk factors in the development of strokes for both young men and women and have a stronger association with the development of strokes in adults younger than 35 years of age.