INTRODUCTION: Sedentary screen time (watching TV or using a computer) predicts cardiovascular outcomes independently from moderate and vigorous physical activity and could affect left ventricular structure and function through the adverse consequences of sedentary behavior. PURPOSE: This study aimed to determine whether sedentary screen time is associated with measures of left ventricular structure and function. METHODS: The Coronary Artery Risk Development in Young Adults Study measured screen time by questionnaire and left ventricular structure and function by echocardiography in 2854 black and white participants, age 43-55 yr, in 2010-2011. Generalized linear models evaluated cross-sectional trends for echocardiography measures across higher categories of screen time and adjusting for demographics, smoking, alcohol, and physical activity. Further models adjusted for potential intermediate factors (blood pressure, antihypertensive medication use, diabetes, and body mass index). RESULTS: The relationship between screen time and left ventricular mass (LVM) differed in blacks versus whites. Among whites, higher screen time was associated with larger LVM (P < 0.001), after adjustment for height, demographics, and lifestyle variables. Associations between screen time and LVM persisted when adjusting for blood pressure, antihypertensive medication use, and diabetes (P = 0.008) but not with additional adjustment for body mass index (P = 0.503). Similar relationships were observed for screen time with LVM indexed to height,2.7 relative wall thickness, and mass-to-volume ratio. Screen time was not associated with left ventricular structure among blacks or left ventricular function in either race group. CONCLUSIONS: Sedentary screen time is associated with greater LVM in white adults, and this relationship was largely explained by higher overall adiposity. The lack of association in blacks supports a potential qualitative difference in the cardiovascular consequences of sedentary screen-based behavior. Copyright © 2013 by the American College of Sports Medicine.