Background and aims Patients with embolic strokes of undetermined source (ESUS) usually present with mild symptoms. We aimed to compare the baseline characteristics between mild and severe ESUS, identify predictors for severe ESUS, and assess outcomes of patients with severe ESUS. Methods In the AF-ESUS (AF-ESUS) dataset, we stratified ESUS severity using the median National Institutes of Health Stroke Scale (NIHSS) score on admission as cut-off. We performed multivariable stepwise regression analyses to identify independent predictors of severe ESUS and to assess the association between ESUS severity and stroke recurrence, death, and new incident atrial fibrillation (AF) on follow-up. The 10-year cumulative probabilities of outcome incidence were estimated by the Kaplan–Meier product limit method. Results In 772 patients (median NIHSS: 6 (interquartile range: 3–12)), 414 (53.6%) patients had severe ESUS (i.e. NIHSS ≥6). Female sex was the only independent predictor for severe ESUS (odds ratio: 1.72 (1.27–2.33)). The rates of recurrence (3.3%/year vs. 3.4%/year, adjusted-hazard ratio: 1.09 (0.73–1.62)) and new incident AF (13.5% vs. 17.0%, adjusted odds ratio: 0.67 (0.44–1.03)) were similar between severe and mild ESUS, but mortality was higher (5.4%/year vs. 3.7%/year, adjusted-hazard ratio: 1.51 (1.05–2.16)) in severe ESUS. The 10-year cumulative probability for stroke recurrence was similar between severe and mild ESUS (38.1% (29.2–48.6) vs. 36.6% (27.8–47.0), log-rank test: 0.01, p = 0.920). The 10-year cumulative probability of death was higher in patients with severe ESUS compared with mild ESUS (40.5% (32.5–50.0) vs. 34.0% (26.0–43.6) respectively; log-rank test: 4.54, p = 0.033). Conclusions Women have more severe ESUS compared with men. Patients with severe ESUS have similar rates of stroke recurrence and new incident AF, but higher mortality compared with mild ESUS.