Importance: International guidelines recommend treatment with statins for patients with preexisting ischemic heart disease to prevent additional cardiovascular events but differ regarding target levels of low-density lipoprotein cholesterol (LDL-C). Trial data on this question are inconclusive and observational data are lacking. Objective: To assess the relationship between levels of LDL-C achieved with statin treatment and cardiovascular events in adherent patients with preexisting ischemic heart disease. Design, Setting, and Participants: Population-based observational cohort study from 2009 to 2013 using data from a health care organization in Israel covering more than 4.3 million members. Included patients had ischemic heart disease, were aged 30 to 84 years, were treated with statins, and were at least 80% adherent to treatment or, in a sensitivity analysis, at least 50% adherent. Patients with active cancer or metabolic abnormalities were excluded. Exposures: Index LDL-C was defined as the first achieved serum LDL-C measure after at least 1 year of statin treatment, grouped as low (≤70.0mg/dL), moderate (70.1-100.0 mg/dL), or high (100.1-130.0mg/dL). Main Outcomes and Measures: Major adverse cardiac events included acute myocardial infarction, unstable angina, stroke, angioplasty, bypass surgery, or all-cause mortality. The hazard ratio of adverse outcomes was estimated using 2 Cox proportional hazards models with low vs moderate and moderate vs high LDL-C, adjusted for confounders and further tested using propensity score matching analysis. Results: The cohort with at least 80% adherence included 31 619 patients, for whom the mean (SD) age was 67.3 (9.8) years. Of this population, 27% were female and 29% had low, 53% moderate, and 18% high LDL-C when taking statin treatment. Overall, there were 9035 patients who had an adverse outcome during a mean 1.6 years of follow-up (6.7 per 1000 persons per year). The adjusted incidence of adverse outcomes was not different between low and moderate LDL-C (hazard ratio [HR], 1.02; 95%CI, 0.97-1.07; P = .54), but it was lower with moderate vs high LDL-C (HR, 0.89; 95%CI, 0.84-0.94; P < .001). Among 54 884 patients with at least 50% statin adherence, the adjusted HR was 1.06 (95%CI, 1.02-1.10; P = .001) in the low vs moderate groups and 0.87 (95%CI, 0.84-0.91; P = .001) in the moderate vs high groups. Conclusions and Relevance: Patients with LDL-C levels of 70 to 100mg/dL taking statins had lower risk of adverse cardiac outcomes compared with those with LDL-C levels between 100 and 130mg/dL, but no additional benefit was gained by achieving LDL-C of 70mg/dL or less. These population-based data do not support treatment guidelines recommending very low target LDL-C levels for all patients with preexisting heart disease.