In recent years, increasing evidence has shown that individuals with Parkinson's disease (PD) have lower levels of 25-hydroxyvitamin D [25(OH) D] relative to healthy controls. We therefore evaluated the relationship between serum 25(OH) D levels and severity in Chinese patients with a Parkinson's disease. From July 2010 to June 2012, consecutive PD patients admitted to the Department of Neurology of General Hospital, Tianjin Medical University, were identified. Clinical information and disease duration were collected. Serum level of 25(OH) D was measured at baseline. PD severity was assessed at admission using the Hoehn and Yahr (HY) stage and Unified Parkinson's Disease Rating Stage (UPDRS) total score. The results indicated that the mean serum 25(OH) D levels were significantly lower in PD patients as compared to normal cases (P < 0.001). There was a negative correlation between levels of 25(OH) D and the disease duration [r (spearman) = -0.124, P = 0.041]. There was a significant negative relationship between circulating serum 25(OH) D levels and severity of Parkinson's disease evaluated by HY stage (P = 0.030) and UPDRS Stage (P = 0.000) even after multivariate adjustment for possible confounders: age, gender, disease duration, levels of Ca, ALP, blood glucose, and seasons. These results suggest that lower 25(OH) D levels may be associated with severity of PD in Chinese patients.