Thirteen cases of autoimmune hepatitis (AIH) were diagnosed from 1990 to 2003 in the area of the Hospital de Sagunto (Valencia, Spain), which attends a population of 112,003 inhabitants aged more than 14 years (54,622 males and 57,381 females). The diagnostic criteria of the International Autoimmune Hepatitis Group were used and patients who, despite having a probable diagnosis of AIH, presented hepatitis C virus infection were excluded. The diagnosis was probable in one patient and definitive in 12. All patients, 11 females and two males aged 45.9 12.2 years (range: 28-66), were classified as AIH type 1. Among the population aged more than 14 years, the mean annual incidence of AIH was 0.83 cases/100,000 inhabitants (95% CI, 0.44-1.42) (range: 0-2.68), showing a significant trend to increase (b = 0.132; p = 0.019). The incidence was higher in women than in men (RR = 5.24; 95% CI, 1.16-23.62). The mean annual incidence was 1.37 (95% CI, 0.68-2.46) (range: 0-3,49) in women and was 0.26 (95% CI, 0.02-0.96) (range: 0-1.83) in men. By age, the maximum mean annual incidence was observed in the group aged 55-64 years (1.6 cases/100,000 inhabitants). The prevalence of AIH in September 2003 was 11.61 cases/100,000 inhabitants aged more than 14 years (95% CI, 6.78-19.86). The prevalence was 3.66 (95% CI, 1-13.35) in men and was 19.17 (95% CI, 10.70-34.33) in women.