Background and Purpose- The HAVOC score (hypertension, age, valvular heart disease, peripheral vascular disease, obesity, congestive heart failure, coronary artery disease) was proposed for the prediction of atrial fibrillation (AF) after cryptogenic stroke. It showed good model discrimination (area under the curve, 0.77). Only 2.5% of patients with a low-risk HAVOC score (ie, 0-4) were diagnosed with new incident AF. We aimed to assess its performance in an external cohort of patients with embolic stroke of undetermined source. Methods- In the AF-embolic stroke of undetermined source dataset, we assessed the discriminatory power, calibration, specificity, negative predictive value, and accuracy of the HAVOC score to predict new incident AF. Patients with a HAVOC score of 0 to 4 were considered as low-risk, as proposed in its original publication. Results- In 658 embolic stroke of undetermined source patients (median age, 67 years; 44% women), the median HAVOC score was 2 (interquartile range, 3). There were 540 (82%) patients with a HAVOC score of 0 to 4 and 118 (18%) with a score of ≥5. New incident AF was diagnosed in 95 (14.4%) patients (28.8% among patients with HAVOC score ≥5 and 11.3% among patients with HAVOC score 0-4 [age- and sex-adjusted odds ratio, 2.29 (95% CI, 1.37-3.82)]). The specificity of low-risk HAVOC score to identify patients without new incident AF was 88.7%. The negative predictive value of low-risk HAVOC score was 85.1%. The accuracy was 78.0%, and the area under the curve was 68.7% (95% CI, 62.1%-73.3%). Conclusions- The previously reported low rate of AF among embolic stroke of undetermined source patients with low-risk HAVOC score was not confirmed in our cohort. Further assessment of the HAVOC score is warranted before it is routinely implemented in clinical practice.