Detection of new atrial fibrillation (AF) after ischemic stroke is challenging. The aim of the TARGET-AF study was to identify relevant markers for ruling out delayed AF in stroke patients. Early and prolonged Holter electrocardiography (ECG) monitoring during hospitalization was performed systematically in consecutive acute stroke patients naive to AF (no history of AF or no AF on baseline ECG). All clinical and paraclinical data for routine etiologic assessment were collected. The diagnostic value of all parameters significantly associated with AF was assessed by comparison of area under the receiver operating characteristic curve (AUC). Of the 300 stroke patients enrolled (mean age, 62.5 ± 15.5 years; sex ratio: 1.7; mean National Institutes of Health Stroke Scale score, 7.1 ± 7.9, median duration of Holter ECG monitoring, 6.8 days), 52 (17.3%) had newly diagnosed AF. Parameters significantly associated with AF were classified by increasing AUC: anterior circulation localization (AUC, 0.604; 95% confidence interval CI, 0.546-0.660), P-wave initial force (AUC, 0.608; 95% CI, 0.545-0.669), left atrial dilatation (AUC, 0.657; 95% CI, 0.600-0.711), National Institutes of Health Stroke Scale score (AUC, 0.667; 95% CI, 0.611-0.720), sex (AUC, 0.683; 95% CI, 0.627-0.736), age (AUC, 0.755; 95% CI, 0.707-0.797), CHA2 DS2 -VASc score (AUC, 0.796; 95% CI, 0.746-0.841), STAF (score for the targeting of AF) score (AUC, 0.842; 95% CI, 0.796-0.882), and plasma brain natriuretic peptide (BNP) level (AUC, 0.868; 95% CI, 0.825-0.904). The use of all parameters combined (AUC, 0.910; 95% CI, 0.872-0.940) was not significantly more efficient in diagnosing AF than BNP alone ( P  = .248). At the Youden plot, the diagnostic properties for BNP >131 pg/mL were sensitivity, 98.1% (95% CI, 89.7-99.7); specificity, 71.4% (95% CI, 65.3-76.9); and negative predictive value, 99.4% (95% CI, 96.9-99.9). Our data indicate that a BNP level ≤131 pg/mL might rule out delayed AF in stroke survivors and could be included in algorithms for AF detection.