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Background: There is a strong interest in PR interval as a predictor for adverse outcomes. However, inconsistent reports have emerged., Objective: The purpose of this study was to test the hypothesis that the significance of PR interval as a predictor depends on the level of contribution of P duration to its length, a contribution that varies across populations., Methods: We tested our hypothesis in 7501 participants from the Third National Health and Nutrition Examination Survey (NHANES III). Participants were divided into two subgroups based on the median P-duration contribution to PR interval (P duration/PR interval * 100). The risk of mortality associated with prolonged (>200 ms) and short (<120 ms) PR interval compared with normal PR interval was examined in all participants and each subgroup., Results: P-duration contribution to the length of PR interval ranged from 30% to 90% (median 70%). During median follow-up of 13.8 years, 2541 deaths occurred. In a multivariable adjusted model, short but not prolonged PR interval was associated with mortality (hazard ratio [HR], (95% confidence interval [CI]): 1.54 (1.18, 2.00) and 1.02 (0.90, 1.16), respectively). However, in a stratified analysis by P-duration contribution to PR interval, both short and prolonged PR interval were associated with mortality in participants with high P-duration contribution (HR (95% CI):1.46 (1.10, 1.94) and 2.00 (1.34, 2.99), respectively) but not in participants with low P-duration contribution (HR (95% CI):1.53 (0.68, 3.41) and 0.99 (0.87, 1.13), respectively); interaction P = .008., Conclusion: PR-interval associations with outcomes are dictated by the level of contribution of P duration to its length, a contribution that has a wide range and is expected to vary across populations. These findings could explain the inconsistent reports of PR-interval associations in different studies and call for caution when using PR interval in risk prediction models., (© 2013 Heart Rhythm Society Published by Heart Rhythm Society All rights reserved.)