Diabetes mellitus can affect ventricular repolarization, and we investigated the impact of diabetes on the risk for cardiac events in older patients with Long QT Syndrome (LQTS). The study population consisted of 1,152 patients with QTc≥450ms who were enrolled in the U.S. portion of the International LQTS Registry and survived beyond 40 years of age. Patients were categorized as having diabetes if they received oral diabetic medication or insulin. End points after age 40 included first cardiac event (syncope, aborted cardiac arrest, sudden cardiac death, whichever occurred first) and all-cause mortality. Follow-up extended from age 41 to 75 years. The risk factors for the end points were evaluated by the Cox model. During follow-up, 193 patients experienced a first cardiac event, and 99 patients died. Among LQTS patients, the development of diabetes in adult LQTS patients was not associated with an increased risk of first cardiac events dominated by syncope. The risk factors for mortality were syncope before age 41, QTc ≥500ms, heart rate >80bpm, and diabetes; there was no mortality interaction involving diabetes and QTc ≥500ms. In conclusion, diabetes and prolonged QTc contributed independent mortality risks in adult patients with LQTS, with no interaction between these two risk factors.