Background/aim: We aimed to examine the seasonal and sex variations in vitamin D levels in children with asthma and their associations with lung function. Materials and methods: We conducted a cohort study involving children aged 7?17 years old with asthma. Vitamin D levels were obtained and pulmonary function tests (PFTs) were performed in winter months (December, January, and February) and at the end of summer (August, September, and October). Seasonal and sex variations in vitamin D levels and lung function were examined. Results: A total of 56 children (26 males, mean age: 11.93 ± 1.8) were enrolled. The mean vitamin D level in winter was 13.36 ± 6.31 ng/mL and increased to 22.89 ± 7.83 ng/mL at the end of summer. Vitamin D levels were significantly lower in the female participants (P = 0.002) in winter. There was no difference in vitamin D levels at the end of the summer between the sexes. No correlations were found between vitamin D levels and PFT parameters in winter or at the end of summer.Conclusion: There are seasonal and sex variations in vitamin D levels in children with asthma. Vitamin D levels do not correlate with lung function