BACKGROUND Decades-old animal experiments suggested that dietary long-chain monounsaturated fatty acids (LCMUFAs) caused cardiotoxicity, leading, for example, development of Canola oil (Canadian oil low in erucic acid) from rapeseed. However, potential cardiotoxicity in humans and contemporary dietary sources of LCMUFAs are unknown. METHODS AND RESULTS We prospectively investigated the associations of plasma phospholipid LCMUFAs (20:1, 22:1, and 24:1), assessed as objective biomarkers of exposure, with incidence congestive heart failure in 2 independent cohorts: 3694 older adults (mean age, 75.2±5.2 years) in the Cardiovascular Health Study (CHS; 1992-2006) and 3577 middle-aged adults (mean age, 54.1±5.8 years) in the Atherosclerosis Risk in Communities Study, Minnesota subcohort (ARIC; 1987-2008). We further examined dietary correlates of circulating LCMUFAs in CHS and ARIC and US dietary sources of LCMUFAs in the 2003-2010 National Health and Nutrition Examination Survey (NHANES). In CHS, 997 congestive heart failure events occurred during 39 238 person-years; in ARIC, 330 events congestive heart failure events occurred during 64 438 person-years. After multivariable adjustment, higher levels of 22:1 and 24:1 were positively associated with greater incident congestive heart failure in both CHS and ARIC; hazard ratios were 1.34 (95% confidence interval, 1.02-1.76) and 1.57 (95% confidence interval, 1.11-2.23) for highest versus lowest quintiles of 22:1, respectively, and 1.75 (95% confidence interval, 1.23-2.50) and 1.92 (95% confidence interval, 1.22-3.03) for 24:1, respectively (P for trend ≤0.03 each). A variety of foods were related to circulating LCMUFAs in CHS and ARIC, consistent with food sources of LCMUFAs in NHANES, including fish, poultry, meats, whole grains, and mustard. CONCLUSIONS Higher circulating levels of 22:1 and 24:1, with apparently diverse dietary sources, were associated with incident congestive heart failure in 2 independent cohorts, suggesting possible cardiotoxicity of LCMUFAs in humans.