Vitamin D deficiency is very frequently observed among young women. Vitamin D deficiency is high in winter because of reduced exposure to sunlight. The main objective of this study was to Assessment 25-hydroxyvitamin D level in Iranian young women in winter and summer. In this cross-sectional study, 4212 men and women_ aged 20 to 35 years, who attended a single-consultation outpatient clinic, were selected. Who participated in this study toward the end of summer, and in winter Serum 25-hydroxy vitamin D (25-OHD) concentrations were measured. T test and X2 were used for data analysis. 4200subjects (66.4% women, 33.6% men) aged 20 to 35 (mean age ± SD 27.11 ± 12.6) participated in the study. The mean 25-(OH) D concentration in summer was 13.41 ± 13, and in winter it was 11.7 ± 11, and the difference was statistically significant (p< 0.02). The prevalence of 25-(OH) vitamin D deficiency was 87.7% in winter and 78.7% in summer (p < 0.05). Low vitamin D status among Young Men and Women during two consecutive winter seasons, improved vitamin D status during the summer was found. Serum 25(OH)D was strongly associated with parameters related to sun exposure, but only weakly with intake of vitamin D supplements.