Background: Vitamin D is an important factor responsible for many physiologic functions. Vitamin D deficiency is associated with increased risk of neurodegenerative disease. The level of vitamin D in Iranian patients with Parkinson's disease and its relationship with severity of symptoms and signs were evaluated in this study.Methods: Eighty-three patients with Parkinson's disease (PD) were recruited using simple non-random sampling. 25-hydroxyvitamin D [25(OH)D3] was measured by Electrochemiluminescence immunoassay (ECLIA). Serum level of calcium and phosphorus was measured to exclude other endocrine disorders.Results: The mean 25(OH)D3 concentration was lower in the PD population than in the normal group. Lower levels of 25(OH)D3 were associated with more severe postural instability and abnormal posture. There was no significant association between levels of 25(OH)D3 and severity of other symptoms of parkinsonism.Conclusion: This analysis showed that serum 25(OH)D levels are lower in PD patients in comparison with normal range. In addition, there was a significant association between the presences of freezing, postural instability and abnormal postures with lower levels of 25(OH)D.