BACKGROUND: In the COMPLETE trial (Complete Versus Culprit-Only Revascularization to Treat Multivessel Disease After Early PCI for STEMI), a strategy of complete revascularization reduced the risk of major cardiovascular events compared with culprit-lesion-only percutaneous coronary intervention in patients presenting with ST-segment–elevation myocardial infarction (STEMI) and multivessel coronary artery disease. Patients with diabetes have a worse prognosis following STEMI. We evaluated the consistency of the effects of complete revascularization in patients with and without diabetes. METHODS: The COMPLETE trial randomized a strategy of complete revascularization, consisting of angiography-guided percutaneous coronary intervention of all suitable nonculprit lesions, versus a strategy of culprit-lesion-only percutaneous coronary intervention (guideline-directed medical therapy alone). In prespecified analyses, treatment effects were determined in patients with and without diabetes on the first coprimary outcome of cardiovascular death or new myocardial infarction and the second coprimary outcome of cardiovascular death, new myocardial infarction, or ischemia-driven revascularization. Interaction P values were calculated to evaluate whether there was a differential treatment effect in patients with and without diabetes. RESULTS: Of the 4041 patients enrolled in the COMPLETE trial, 787 patients (19.5%) had diabetes. The median HbA1c (glycated hemoglobin) was 7.7% in the diabetes group and 5.7% in the nondiabetes group. Complete revascularization consistently reduced the first coprimary outcome in patients with diabetes (hazard ratio, 0.87 [95% CI, 0.59–1.29]) and without diabetes (hazard ratio, 0.70 [95% CI, 0.55–0.90]), with no evidence of a differential treatment effect (interaction P =0.36). Similarly, for the second coprimary outcome, no differential treatment effect (interaction P =0.27) of complete revascularization was found in patients with diabetes (hazard ratio, 0.61 [95% CI, 0.43–0.87]) and without diabetes (hazard ratio, 0.48 [95% CI, 0.39–0.60]). CONCLUSIONS: Among patients presenting with STEMI and multivessel disease, the benefit of complete revascularization over a culprit-lesion-only percutaneous coronary intervention strategy was consistent regardless of the presence or absence of diabetes.