['25-Hydroxyvitamin D [25(OH)D] has an important role in fibrosis progression and inflammatory response in patients with various etiologies of chronic liver disease. However, its influence on autoimmune hepatitis (AIH) has not been investigated. We evaluated the association of serum 25(OH)D levels with clinical, biochemical and histological features and response to therapy in AIH.', 'Serum 25(OH)D levels were quantified in 68 therapy naïve AIH patients and 34 healthy controls.', 'Mean serum 25(OH)D levels were significantly lower in AIH compared to healthy controls (16.8 ± 9.2 vs. 35.7 ± 13.6, p < 0.0001). Low levels of 25(OH)D (<30 µg/L) were independently associated with advance fibrosis and severe interface hepatitis in AIH patients [p = 0.014; odds ratio (OR) 0.12, 95% confidence interval (CI) 0.02-0.65 and p = 0.020; OR 0.17, 95% CI 0.04-0.76, respectively]. Severe 25(OH)D deficiency (<10 µg/L) was associated with advance fibrosis, severe interface hepatitis, low platelet counts and sampling time in a univariate analysis. Only interface hepatitis and fibrosis scores were independently associated with 25(OH)D deficiency in a multiple regression analysis (p = 0.005; OR 0.12, 95% CI 0.03-0.53 and p = 0.022; OR 0.15, 95% CI 0.03-0.75, respectively). Mean serum 25(OH)D levels were lower in non-responders compared to responders (9.2 ± 4.8 vs. 17.1 ± 9.4, p = 0.015), and 25(OH)D deficiency was more commonly observed in non-responders than the responders (80 vs. 43%, p = 0.036).'] Are low serum vitamin D levels associated with severe histological features and poor response to therapy in patients with autoimmune hepatitis?