Vitamin D has been shown to have immunomodulatory and anti-inflammatory properties in addition to its well-established role in the maintenance of mineral homeostasis and bone health. The aims of this study were to evaluate vitamin D status in patients with juvenile idiopathic arthritis (JIA), and also to examine whether there is an association between serum levels of 25-hydroxyvitamin D [25(OH)D] and disease activity in JIA. Children with JIA who had an outpatient visit between March and April 2011 were evaluated retrospectively. Clinical and laboratory findings and vitamin D levels were evaluated. Disease activity was calculated using JADAS-27. Serum vitamin D levels were measured using high-performance liquid chromatography (HPLC). A total of 47 patients, 29 (61.7%) of them girls, with a mean age of 9.3±3.9 years and a median follow-up period of 28 months, were included in the study. The mean serum vitamin D level of all patients was 17.7±11.6 ng/ml. Vitamin D insufficiency (serum vitamin D: 15-20 ng/ml) and deficiency (serum vitamin D level <15 ng/ml) were found in 9 (19.1%) and 25 patients (53.2%), respectively. The vitamin D level was <20 ng/ml in 72.3% of the children. Only 13 patients (27.7%) were found to have adequate vitamin D levels (>20 ng/ml). There was a significant negative correlation between vitamin D levels and disease activity (p=0.01, r=-0,37). The mean JADAS-27 score was significantly higher in patients with 25(OH)D levels <15 ng/ml than in patients with 25(OH)D levels >15 ng/ml (p = 0.003). We suggest that vitamin D deficiency may be a possible modifiable risk factor affecting disease activity in JIA.