Aim: The role of vitamin D in individuals with chronic hepatitis B (CHB) is unclear. We aimed to explore the association of baseline vitamin D level with genetic determinants and week-104 treatment outcome in CHB patients. Methods: Baseline serum 25-hydroxycholecalciferol (25(OH)D) levels and genetic polymorphism within GC, DHCR7, and CYP2R1 were determined in stored serum of 560 patients who were enrolled into amulticenter, randomized, controlled study and completed 104 weeks of telbivudine monotherapy or telbivudine-based optimized therapy. Virologic response was defined as hepatitis B virus DNA <300 copies/mL (52 IU/mL) at week 104. Results: The mean 25(OH)D value was 29.64 ng/mL. The percentage of patients with vitamin D insufficiency (<30 ng/mL) and vitamin D deficiency (<20 ng/mL) were 55.0% and 20.9%, respectively. Gender, season, latitude, and GC rs2282679 polymorphism were independent factors of vitamin D status. Patients with sufficient vitamin D (>= 30 ng/mL) achieved a higher virologic response rate than those with vitamin D insufficiency (81.7% vs. 67.2%, P < 0.001). The area under the curve of 25(OH)D to predict virologic response was 0.65 (P < 0.001; 95% confidence interval, 0.62-0.67). On multivariate analysis, 25(OH)D level was an independent predictor of virologic response, but not associated with hepatitis B envelope antigen (HBeAg) seroconversion or alanine aminotransferase (ALT) normalization. Conclusions: Vitamin D insufficiency was highly prevalent in treatment-naive CHB patients in mainland China. Latitude and genetic determinants affect vitamin D status. Baseline vitamin D level can predict week-104 virologic response, but not HBeAg seroconversion or ALT normalization.