In recent years, increasing evidence has shown that children with autism spectrum disorders (ASDs) have lower levels of 25-hydroxyvitamin D [25(OH) D] relative to healthy controls. The purpose of this study was to evaluate the serum 25(OH) D levels in Chinese children with ASD. From January 2012 to December 2012, consecutive patients with ASD admitted to the Department of Neurology were identified. Clinical information was collected. Serum levels of 25(OH) D were measured at baseline. ASD severity was assessed at admission using the Childhood Autism Rating Scale total score. The results indicated that the mean serum 25(OH) D levels were significantly lower in autistic children as compared with normal cases (P=0.002). There was a significant negative relationship between circulating serum 25(OH) D levels and the severity of autism evaluated according to Childhood Autism Rating Scale Scores (P=0.000), after adjustment for the possible covariates such as age, sex, BMI, serum levels of calcium, phosphate, and magnesium, and seasons. After adjusting for all other possible covariates, 25(OH) D levels that remained can be seen as an independent predictor of ASD with an adjusted odds ratio of 1.23 (95% confidence interval, 1.10-1.37). These results indicate that lower 25(OH) D levels may be independently associated with severity of ASD among Chinese patients, and lower serum 25(OH) D levels could be considered as an independent risk factor for ASD.