To assess the safety and efficacy of deferred versus complete revascularization using a fractional flow reserve (FFR)–guided strategy in patients with diabetes mellitus (DM), we analyzed all DM patients who underwent FFR-guided revascularization from January 1, 2010, to December 12, 2013. Patients were divided into 2 groups: those with ≥1 remaining FFR-negative (>0.80) medically treated lesions [FFR(−)MT] and those with only FFR-positive lesions (≤0.80) who underwent complete revascularization [FFR(+)CR] and were followed until July 1, 2015. The primary end point was the incidence of major adverse cardiovascular events (MACE), a composite of death, myocardial infarction (MI), target lesion (FFR assessed) revascularization, and rehospitalization for acute coronary syndrome. A total of 294 patients, 205 (69.7%) versus 89 (30.3%) in FFR(−)MT and FFR(+)CR, respectively, were analyzed. At a mean follow-up of 32.6 ± 18.1 months, FFR(−)MT was associated with higher MACE rate 44.0% versus 26.6% (log-rank p = 0.02, Cox regression–adjusted hazard ratio [HR] 2.01, 95% confidence interval [CI] 1.21 to 3.33, p <0.01), and driven by both safety and efficacy end points: death/MI (HR 2.02, 95% CI 1.06 to 3.86, p = 0.03), rehospitalization for acute coronary syndrome (HR 2.06, 95% CI 1.03 to 4.10, p = 0.04), and target lesion revascularization (HR 3.38, 95% CI 1.19 to 9.64, p = 0.02). Previous MI was a strong effect modifier within the FFR(−)MT group (HR 1.98, 95% CI 1.26 to 3.13, p <0.01), whereas this was not the case in the FFR(+)CR group (HR 0.66, 95% CI 0.27 to 1.62, p = 0.37). Significant interaction for MACE was present between FFR groups and previous MI (p = 0.03). In conclusion, in patients with DM, particularly those with previous MI, deferred revascularization is associated with poor medium-term outcomes. Combining FFR with imaging techniques may be required to guide our treatment strategy in these patients with high-risk, fast-progressing atherosclerosis.