Background: Despite the risk of critical heart disease, poor adherence to treatment is common in patients with lifestyle-related diseases such as hypercholesterolemia. The association between adherence to treatment and clinical outcome was examined in JELIS (Japan EPA Lipid Intervention Study) and strategies for avoiding poor adherence were explored. Methods and Results: Patients taking 80% or more of the study medications were considered to exhibit good adherence. The primary endpoint was either sudden cardiac death or myocardial infarction. Adherence was lower in the eicosapentaenoic acid (EPA) + statin group (66.5%) than in the statin alone group (72.5%). In good adherers with previous coronary artery disease, EPA substantially reduced the risk compared with statin alone (hazard ratio 0.55, 95% confidence intervals 0.34-0.88, P<0.014). Furthermore, the clinical benefit of EPA + statin was significantly larger in patients with good adherence than in those with poor adherence (P=0.041). Finally, a 5-year risk prediction model constructed from the data indicated that complete adherence would lead to 51% reduction of risk compared with non-adherence. Conclusions: Good adherence to medication was associated with a lower cardiovascular risk than with poor adherence, and the assistance of a pharmacist is of great importance in achieving persistent adherence during treatment. (Circ J 2010; 74: 510 - 517)