Introduction: Chronic kidney disease (CKD) is characterized by alteration of mineral metabolism of calcium and phosphorus related to phosphate retention, secondary hyperparathyroidism and impairment of vitamin D hydroxylation in the kidneys. Additionally, bone disorders can be augmented by vitamin D deficiency in the diet. Study aimed at investigation of 25-hydroxyvitamin D [25(OH)D] serum level in children with CKD treated in Pediatric Nephrology Ward, its relationship with blood pressure values, selected laboratory parameters values and evaluation of seasonal and sex related differences. Material and methods: The study group included 28 children with CKD in stage 2-5 (10 girls and 18 boys; mean age 9.5 ±5.3 years), all of them were treated conservatively. The main cause of CKD in examined children were congenital abnormalities of the kidney and urinary tract (CAKUT). Mean estimated GFR was 40.3 ±23.1 ml/min/1.73 m 2. Seventy-five percent of patients were supplemented with vitamin D formulas. The evaluation of serum vitamin D was conducted during the routine visit in Paediatric Nephrology Ward. Serum level of 25(OH)D was measured by using an electrochemiluminescence method. Results: In the examined group, mean height was 126 ±29 cm, and 29% of children were under 3 rd percentile for height. The mean haemoglobin level was 12.4 ±1.9 g/dl; mean creatinine level 181.2 ±133.6 µmol/l, phosphate level 1.5 ±0.2 mmol/l; total calcium level 2.4 ±0.1 mmol/l, mean PTH level 84.5 ±62.3 pg/ml. Mean 25(OH)D serum level in CKD group was 38.3 ±14.9 ng/ml and was significantly higher than in control group (23.3 ±11.3 ng/ml; p < 0.0001). Conclusions: According to current standards of serum vitamin D level in children in Poland 61% of patients with CKD from study group had optimal vitamin D serum level without seasonal and sex related differences.