IMPORTANCE It is uncertain whether lipoprotein(a) [Lp(a)], which is associated with incident cardiovascular disease, is an independent risk factor for recurrent cardiovascular events after acute coronary syndrome (ACS). OBJECTIVE To determine the association of Lp(a) concentration measured after ACS with the subsequent risk of ischemic cardiovascular events. DESIGN, SETTING, AND PARTICIPANTS This nested case-cohort analysiswas performed as an ad hoc analysis of the dal-Outcomes randomized clinical trial. This trial compared dalcetrapib, the cholesteryl ester transfer protein inhibitor, with placebo in patients with recent ACS and was performed between April 2008 and September 2012 at 935 sites in 27 countries. There were 969 case patients who experienced a primary cardiovascular outcome, and there were 3170 control patients who were event free at the time of a case event and had the same type of index ACS (unstable angina ormyocardial infarction) as that of the respective case patients. Concentration of Lp(a) was measured by immunoturbidimetric assay. Data analysis for this present study was conducted from June 8, 2016, to April 21, 2017. INTERVENTIONS Patients were randomly assigned to receive treatment with dalcetrapib, 600mg daily, or matching placebo, beginning 4 to 12 weeks after ACS. MAIN OUTCOMES AND MEASURES Death due to coronary heart disease, a major nonfatal coronary event (myocardial infarction, hospitalization for unstable angina, or resuscitated cardiac arrest), or fatal or nonfatal ischemic stroke. RESULTS The mean (SD) age was 63 (10) years for the 969 case patients and 60 (9) years for the 3170 control patients, and both cohorts were composed of predominantly male (770 case patients [79%] and 2558 control patients [81%]; P = .40) and white patients (858 case patients [89%] and 2825 control patients [89%]; P = .62). At baseline, the median (interquartile range) Lp(a) level was 12.3 (4.7-50.9)mg/dL. There was broad application of evidence-based secondary prevention strategies after ACS, including use of statins in 4030 patients (97%). The cumulative distribution of baseline Lp(a) levels did not differ between cases and controls at P = .16. Case-cohort regression analysis showed no association of baseline Lp(a) level with risk of cardiovascular events. For a doubling of Lp(a) concentration, the hazard ratio (case to control) was 1.01 (95%CI, 0.96-1.06; P = .66) after adjustment for 16 baseline variables, including assigned study treatment. CONCLUSIONS AND RELEVANCE For patients with recent ACS who are treated with statins, Lp(a) concentration was not associated with adverse cardiovascular outcomes. These findings call into question whether treatment specifically targeted to reduce Lp(a) levels would thereby lower the risk for ischemic cardiovascular events after ACS. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT00658515.