OBJECTIVE To study whether ischemic heart disease (IHD) mortality among patients with type 1 diabetes (T1D) depends on the age at onset of diabetes and whether this effect is sex specific. RESEARCH DESIGN AND METHODS The study examined long-term IHD-specific mortality in a Finnish population-based cohort of patients with early-onset (0-14 years) and late-onset (15-29 years) T1D (n = 17,306). RESULTS During 433,782 person-years of follow-up, 478 deaths from IHD were observed. Within the early-onset cohort, the average crude mortality rate in women was 33.3% lower than in men, whereas in the late-onset cohort, mortality was only one-half that in men. In contrast, the standardized mortality ratio (SMR) was higher in women than in men (21.6 [95% CI 17.2-27.0] vs. 5.8 [5.1-6.6]). The difference in SMR between sexes was more striking in the early-onset cohort (women 52.8 [36.3-74.5], men 12.1 [9.2-15.8]). The SMR was also greater in women in the late-onset cohort (15.8 [11.8-20.7]) compared with men (5.0 [4.3-5.8]). The relative risk of dying from IHD was greatest in women aged <40 years and 40-60 years in the early- and late-onset cohorts, respectively. CONCLUSIONS The risk of mortality from IHD is exceptionally high in women with early-onset T1D compared with women in the background population. These observations underscore the importance of identifying risk factors early in women and delivering more-aggressive treatment after diagnosis.