Background: Apo CIII (apolipoprotein CIII) is an important regulator of triglyceride metabolism and was associated with cardiovascular risk in several cohorts. It is present in 4 major proteoforms, a native peptide (CIII_0a), and glycosylated proteoforms with zero (CIII_0b), 1 (CIII₁, most abundant), or 2 (CIII₂) sialic acids, which may differentially modify lipoprotein metabolism. We studied the relationships of these proteoforms with plasma lipids and cardiovascular risk. Methods: Apo CIII proteoforms were measured by mass spectrometry immunoassay in baseline plasma samples of 5791 participants of Multi-Ethnic Study of Atherosclerosis, an observational community-based cohort. Standard plasma lipids were collected for up to 16 years and cardiovascular events (myocardial infarction, resuscitated cardiac arrest, or stroke) were adjudicated for up to 17 years. Results: Apo CIII proteoform composition differed by age, sex, race and ethnicity, body mass index, and fasting glucose. Notably, CIII₁was lower in older participants, men and Black and Chinese (versus White) participants, and higher in obesity and diabetes. In contrast, CIII₂was higher in older participants, men, Black, and Chinese persons, and lower in Hispanic individuals and obesity. Higher CIII₂to CIII₁ratio (CIII₂/III₁) was associated with lower triglycerides and higher HDL (high-density lipoprotein) in cross-sectional and longitudinal models, independently of clinical and demographic risk factors and total apo CIII. The associations of CIII_0a/III₁and CIII_0b/III₁with plasma lipids were weaker and varied through cross-sectional and longitudinal analyses. Total apo CIII and CIII₂/III₁were positively associated with cardiovascular disease risk (n=669 events, hazard ratios, 1.14 [95% CI, 1.04-1.25] and 1.21 [1.11-1.31], respectively); however, the associations were attenuated after adjustment for clinical and demographic characteristics (1.07 [0.98-1.16]; 1.07 [0.97-1.17]). In contrast, CIII_0b/III₁was inversely associated with cardiovascular disease risk even after full adjustment including plasma lipids (0.86 [0.79-0.93]). Conclusions: Our data indicate differences in clinical and demographic relationships of apo CIII proteoforms, and highlight the importance of apo CIII proteoform composition in predicting future lipid patterns and cardiovascular disease risk.