Autoimmune hepatitis is classically a disease of young women. Our aims were to determine its occurrence, clinical phenotype, and outcome in elderly patients and contrast findings to young adults. Two-hundred-and-five white North American adults with definite type 1 autoimmune hepatitis were grouped according to age at presentation and the groups compared. Forty-seven patients (23%) were aged >60 years (median age, 68 years), and 31 patients (15%) were aged <30 years (median age, 25 years). The patients >60 years had a higher frequency of cirrhosis at presentation than the patients <30 years (33% versus 10%, P ؍ .03). They also had thyroid or rheumatic diseases more commonly (42% vs. 13%, P ؍ .006). HLA DR3 occurred more frequently in the patients <30 years than in those >60 years (58% vs. 23%, P ؍ .004), and HLA DR4 occurred more often in the patients >60 years (47% vs. 13%, P ؍ .003). Patients aged >60 years failed corticosteroid treatment less commonly than those aged <30 years (5% vs. 24%, P ؍ .03). Autoimmune hepatitis occurred in patients aged 18-30 years (15%), 31-39 years (15%), 40-49 years (21%), 50-59 years (25%), and >60 years (23%). Differences in age distribution, HLA frequencies, and treatment outcome occurred after age >40 years. In conclusion, elderly patients have a greater frequency of cirrhosis at presentation and HLA DR4 than patients <30 years, and they have a lower occurrence of treatment failure. Transitions in clinical and genetic phenotypes occur after age >40 years. Genetic susceptibilities may favor etiologic factors that are age-related. (HEPATOLOGY 2006;43:532-538.)