Background We aimed to assess the prevalence and degree of overlap of potential embolic sources (PES) in patients with embolic stroke of undetermined source (ESUS). Methods and Results In a pooled data set derived from 3 prospective stroke registries, patients were categorized in ≥1 groups according to the PES that was/were identified. We categorized PES as follows: atrial cardiopathy, atrial fibrillation diagnosed during follow-up, arterial disease, left ventricular disease, cardiac valvular disease, patent foramen ovale, and cancer. In 800 patients with ESUS (43.1% women; median age, 67.0 years), 3 most prevalent PES were left ventricular disease, arterial disease, and atrial cardiopathy, which were present in 54.4%, 48.5%, and 45.0% of patients, respectively. Most patients (65.5%) had >1 PES, whereas only 29.7% and 4.8% of patients had a single or no PES, respectively. In 31.1% of patients, there were ≥3 PES present. On average, each patient had 2 PES (median, 2). During a median follow-up of 3.7 years, stroke recurrence occurred in 101 (12.6%) of patients (23.3 recurrences per 100 patient-years). In multivariate analysis, the risk of stroke recurrence was higher in the atrial fibrillation group compared with other PES, but not statistically different between patients with 0 to 1, 2, or ≥3 PES. Conclusions There is major overlap of PES in patients with ESUS. This may possibly explain the negative results of the recent large randomized controlled trials of secondary prevention in patients with ESUS and offer a rationale for a randomized controlled trial of combination of anticoagulation and aspirin for the prevention of stroke recurrence in patients with ESUS. Clinical Trial Registration URL: http://www.clinicaltrials.gov. Unique identifier: NCT02766205.