Purpose: To evaluate the serum 25-hydroxy vitamin D, vitamin B12, and folic acid (FA) levels in progressive and nonprogressive patients with keratoconus (KCN). Methods: Fifty-five patients with KCN (28 progressive and 27 nonprogressive) who were followed up for at least 12 months were recruited and separated into 2 groups according to corneal topographic progression criteria. Age- and sex-matched 30 healthy individuals formed the control group. Serum vitamin D, B12, FA, and Ca levels were compared among control, nonprogressive, and progressive groups. The effect of gender, age, vitamin D, B12, and FA parameters on KCN progression was investigated. Results: Serum vitamin D levels were 12.77 6 5.52, 11.78 6 4.32, and 17.40 6 5.12 ng/mL in nonprogressive, progressive, and control groups, respectively. The serum vitamin D levels in KCN groups (nonprogressive and progressive) were significantly lower than the control group (P = 0.002 and P, 0.001, respectively). There was no significant difference between all groups in serum vitamin B12, FA, and Ca levels (P. 0.05). Decreased vitamin D levels significantly increased nonprogressive KCN probability 1.23 times and progressive KCN probability 1.29 times more than the control group (P = 0.002, 95% confidence interval, 0.708-0.925; P, 0.001, 95% confidence interval, 0.668-0.888, respectively). Conclusions: Serum vitamin D levels were significantly decreased in both KCN groups. However, serum vitamin B12, FA, and Ca levels were similar in all groups. Serum vitamin D evaluation of patients with KCN at onset and follow-up examinations may help to predict the course of the disease.