BACKGROUND & AIMS:: The historical prevalence and long-term outcome of undiagnosed celiac disease (CD) are unknown. We investigated the long-term outcome of undiagnosed CD and whether the prevalence of undiagnosed CD has changed during the past 50 years. METHODS:: This study included 9,133 healthy young adults at Warren Air Force Base (sera were collected between 1948 and 1954) and 12,768 sex-matched subjects from 2 recent cohorts from Olmsted County, Minnesota, with either similar years of birth (n=5,558) or age at sampling (n=7,210) to that of the Air Force cohort. Sera were tested for tissue transglutaminase and, if abnormal, for endomysial antibodies. Survival was measured during a follow-up period of 45 years in the Air Force cohort. The prevalence of undiagnosed CD between the Air Force cohort and recent cohorts was compared. RESULTS:: Of 9,133 persons from the Air Force cohort, 14 (0.2%) had undiagnosed CD. In this cohort, during 45 years of follow-up, all-cause mortality was greater in persons with undiagnosed CD than among those who were seronegative (hazard ratio=3.9; 95% CI, 2.0-7.5; P <.001). Undiagnosed CD was found in 68 (0.9%) persons with similar age at sampling and 46 (0.8%) persons with similar years of birth. The rate of undiagnosed CD was 4.5-fold and 4-fold greater in the recent cohorts (respectively) than in the Air Force cohort (both P=.0001). CONCLUSIONS:: During 45 years of follow-up, undiagnosed CD was associated with a nearly 4-fold increased risk of death. The prevalence of undiagnosed CD appears to have increased dramatically in the United States during the past 50 years.