Vitamin D plays an important role in regulation of innate and acquired immunity. The aim of the study was to assess the serum concentration of a vitamin D metabolite – 25(OH)D – in children with recurrent respiratory tract infections. Two hundred and ninety-six children with recurrent infections, aged 1–18 years, who were hospitalised in the Department of Paediatrics and Allergy for an extended diagnosis of recurrent infections were recruited. The hepatic metabolite of vitamin D, namely 25(OH)D was determined in the analysed sera in different seasons of the year. Sixty-one per cent of the children had a low concentration of 25(OH)D. The vitamin D level was shown to be inversely correlated with the age of the patient; the younger the child, the higher the serum concentration of calcidiol. There was no correlation between the concentration of 25(OH)D and gender or season of the year in which the measurements were done. The results showed that vitamin D deficiency (serum concentration below 30 ng/mL) is common in children with recurrent respiratory tract infections from the region of Łódź.