The circulating 25-hydroxylated form of vitamin D3, 25(OH)D, and serum ferritin concentrations have been described to be associated with disease progression in chronic hepatitis C. Both parameters also have been assessed with regard to treatment outcome, however, with divergent results. This study examined both the pre- and posttreatment serum concentrations of 25(OH)D and ferritin in 191 patients infected chronically with hepatitis C virus (HCV) type 1 with regard to liver inflammatory activity (grading), disease progression in terms of fibrosis (staging) and an antiviral treatment outcome. Mean pretreatment serum 25(OH)D and ferritin concentrations were 18 ± 10 ng/ml and 280 ± 225 μg/L, respectively. Multivariate analysis revealed lower pretreatment serum 25(OH)D and higher ferritin concentrations to be significantly related to both severity of inflammatory activity and of fibrotic alterations. Pretreatment serum ferritin concentration, furthermore, unlike 25(OH)D concentration, was found to be associated with a sustained virological response by uni- and multivariate analyses. A sustained virological response was featured by a significant increase in serum 25(OH)D levels (18 ± 10 ng/ml vs. 22 ± 11 ng/ml; P < 0.01), a reduction of serum ferritin concentration (191 ± 156 μg/L vs. 103 ± 63 μg/L; P < 0.001) and a normalization of serum alanine aminotransferase (ALT) and γ-glutamyl-transferase (γ-GT) activities. Taken together, decreased 25(OH)D and increased ferritin serum levels indicate the severity of hepatic inflammation and fibrosis in patients infected chronically with HCV type 1. Elevated ferritin, furthermore, was found to be an independent predictor for standard IFN-based therapy responsiveness. J. Med. Virol. 85:1534–1541, 2013. © 2013 Wiley Periodicals, Inc.