Background: Lipoprotein(a) [Lp(a)] can be reported as mass concentration (mg/dL) or particle concentration (nmol/L). Because Lp(a) biology is likely mediated at the particle level, molar concentration may better reflect biologically relevant exposure. We compared molar- and mass-based Lp(a) measurements in relation to cardiovascular outcomes, LPA genetic variation, and apo(a) isoform phenotype in a population-based cohort. Methods: Lp(a) was measured in 6,182 participants in the Young Finns Study. Among participants aged [≥]40 years, associations with prevalent coronary artery disease (CAD) and composite cardiovascular disease (CVD) were assessed using logistic regression models. Lp(a) was modeled as quintiles, inverse-normal transformed continuous variables, and clinically relevant cut-points. Discrimination and model fit were evaluated using c-statistics and Akaike?s Information Criterion. Associations with an LPA genetic risk score (GRS) and apo(a) isoform phenotype were examined using Spearman?s correlation analyses. Results: The relation between molar and mass Lp(a) was not constant across the concentration range. The molar-to-mass ratio increased across higher clinically relevant Lp(a) categories, indicating concentration-dependent correspondence between nmol/L and mg/dL. The molar-to-mass ratio was directly associated with the LPA GRS and inversely associated with apo(a) isoform size, and these associations were more strongly attenuated after adjustment for molar than for mass Lp(a). Across CAD and composite CVD, molar- and mass-based Lp(a) showed broadly similar associations. In fully adjusted CAD models, the odds ratio for the highest versus lowest quintile was 1.55 for molar Lp(a) and 1.67 for mass Lp(a); corresponding continuous-model ORs were 1.20 and 1.22 per 1-unit increase in inverse-normal transformed Lp(a). Discrimination and global model fit were essentially identical between the two measurement scales. Conclusions: Molar- and mass-based Lp(a) measurements showed comparable epidemiologic associations with prevalent cardiovascular outcomes. However, molar reporting aligned somewhat more closely with the genetic and structural determinants of Lp(a), supporting continued standardization toward particle-based reporting.