Background— We aimed to determine the prevalence of silent myocardial infarction (SMI) in people with newly diagnosed type 2 diabetes mellitus and its relationships to future myocardial infarction (MI) and all-cause mortality. Methods and Results— We examined data from the 5102 patients in the 30-year UK Prospective Diabetes Study (UKPDS) and used Cox proportional hazards regression to examine outcomes by SMI status. Of 1967 patients with complete baseline data, 326 (16.6%) had ECG evidence of SMI (Minnesota codes 1.1 or 1.2) at enrollment. Those with SMI were more likely to be older, female, sedentary, and nonsmokers compared with those without SMI. Their mean blood pressure was greater despite more intensive antihypertensive treatment; they were more likely to be taking aspirin and lipid-lowering therapy; and they had a greater prevalence of microangiopathy. Fully adjusted hazard ratios for those with versus those without SMI in multivariate models that included UKPDS Risk Engine variables were 1.58 (95% confidence interval, 1.22–2.05) for fatal MI and 1.31 (95% confidence interval, 1.10–1.56) for all-cause mortality. Hazard ratios for first fatal or nonfatal MI and for first nonfatal MI were nonsignificant. The net reclassification index showed no improvement when SMI was added to these models, and the integrated discrimination index showed that SMI marginally improved the prediction of fatal MI and all-cause mortality. Conclusions— About 1 in 6 UKPDS patients with newly diagnosed type 2 diabetes mellitus had evidence of SMI, which was independently associated with an increased risk of fatal MI and all-cause mortality. However, identification of SMI does not add substantively to current UKPDS Risk Engine predictive variables. Clinical Trial Registration— URL: http://www.controlled-trials.com . Identifier: ISRCTN number 75451837.