BACKGROUND: The Thrombosis Prevention Trial was a primary prevention factorial trial that reported a reduction in the risk of coronary heart disease (CHD) with warfarin and/or aspirin. This article examines compliance (duration of treatment) with warfarin treatment and whether warfarin has a retained effect. METHODS: Risk of CHD while complying with warfarin treatment was compared with risk of CHD in all participants randomized to placebo. Simultaneously, risk of CHD in ex-warfarin users was compared with controls receiving placebo to determine the possibility of a retained effect. A second analysis, preserving the advantage of randomization, estimated the potential increase in the time to a CHD event in patients randomized to active treatment compared with patients randomized to placebo, if all patients in both active and placebo groups had fully complied with the trial treatment. RESULTS: Risk of all CHD while complying with warfarin treatment was associated with a hazard ratio (HR) of 0.75 (95% confidence interval [CI], 0.60-0.94), which was lower than the HR obtained by intention-to-treat analysis (0.79; 95% CI, 0.65-0.96). Regarding fatal cases of CHD, the HR was 0.49 (95% CI, 0.32-0.75) while compliant with warfarin treatment, which is also lower than the HR obtained by intention-to-treat analysis (0.61, 95% CI, 0.43-0.85). Ex-warfarin users had a retained risk reduction of 23% for all CHD (0.77; 95% CI, 0.58-1.02) and of 34% for fatal events (0.66; 95% CI, 0.41-1.04). Expected survival time to a CHD event if patients randomized to warfarin had fully complied with treatment was 1.39 times greater (95% CI, 1.12-1.69) than if patients randomized to placebo had fully complied with placebo, whereas for fatal CHD the relative increase in survival time was 2.04 times greater for the former (95% CI, 1.43-2.86). CONCLUSIONS: Full compliance with warfarin treatment may lower by 50% the risk of fatal CHD. There is also evidence of a retained effect. These results strengthen previous evidence of the potential benefits of low-intensity oral anticoagulation with warfarin.