Vitamin D is a hormone that affects various systems of the body. In addition to the known effect of vitamin D on the musculoskeletal system, recent epidemiologic studies associate low serum vitamin D levels with an increased risk for numerous diseases, including cancers (breast and colorectal cancers), autoimmune diseases (multiple sclerosis and type 1 DM), skin diseases (psoriasis, vitiligo, morphea, hyperkeratotic palmoplantar eczema, and acanthosis nigricans), infectious diseases (tuberculosis, otitis media, upper/lower respiratory tract infections, and influenza infection), type 2 DM, hypertension, cardiovascular diseases, and obesity. This is probably because of the anti-inflammatory and immunomodulatory characteristics of vitamin D, and its possible effects on cytokine levels. Presence of vitamin D receptors in the thymus and peripheral T cells shows vitamin D has important effects on T-cell development and activity. Active vitamin D has a direct effect on T cells and antigen-presenting cells, and it also suppresses the differentiation of B cells to plasma cells and inhibits immunoglobulin production (1, 2).Although numerous studies have investigated the clinical role of vitamin D in chronic infectious diseases, there are few studies that reflect the importance of vitamin D, particularly for the prognosis of inflammatory diseases that acutely affect the liver such as acute toxic, ischemic, or viral hepatitis. It is not known how serum 25-hydroxy (OH) vitamin D levels are affected in acute liver injuries. In addition, it is unknown whether serum vitamin D levels change parallel to the recovery of clinical and biochemical test parameters in patients with acute hepatitis. This study aimed to detect serum vitamin D levels in patients with acute hepatitis and determine whether serum vitamin D levels are associated with acute toxic, ischemic, or viral hepatitis. We also investigated whether serum vitamin D levels contributed to prognosis and/or mortality of patients with acute hepatitis. Objective: Vitamin D is a key regulator of calcium homeostasis and has anti-inflammatory and immunomodulatory effects. Active vitamin D has a direct effect on T cells and antigen-presenting cells. It also suppresses the differentiation of B cells to plasma cells and inhibits immunoglobulin production. Vitamin D supplementation is associated with a favorable outcome in chronic inflammatory diseases. In this study, we aimed to determine serum vitamin D levels in patients with acute toxic, ischemic, or viral hepatitis and whether the levels had an effect on clinical progression or mortality in patients with acute hepatitis. Methods:Forty-eight patients (26 men and 22 women) and 35 controls (16 men and 19 women) aged >18 years who were diagnosed as having acute hepatitis and hospitalized in the Internal Medicine Department were enrolled. To determine serum 25-hydroxy (OH) vitamin D levels, two fasting blood samples, first in the initial 24-48 h following hospitalization and second on the day of discharge, were obtained from the part...