The study objective was to assess serum and urine fibronectin (FN) levels in children with vesicoureteral reflux (VUR) depending on reflux grade and urine osmolality. The study group (1) consisted of 54 VUR children, median age 4.28 (range 0.6-15) years: subgroup A, 19 children with grade II; subgroup B, 19 with grade III; and subgroup C, 16 with grade IV or V VUR. The control group (2) included 27 healthy children. The immunoenzymatic method enzyme immunoassay (EIA) was used to determine serum soluble and urine FN levels, with an osmometer to measure urinary osmolality. The median urine FN in VUR children was 224.1 (15.4-3537) ng/mg creatinine (Cr), compared with the control group: 137.9 (20.3-670.6) ng/mg Cr (p<0.05), whereas median serum FN was 395.0 (13.0-779.9) ng/ml and 121.9 (25-345.1) ng/ml (p<0.05), respectively. A detailed analysis showed that only in subgroup C was the level of urinary FN significantly higher than in the control group (p<0.01). However, serum concentration was elevated in all VUR children (A-C) compared with controls (p<0.01). Reduced osmolality, below 800 mOsm/kg H2O, was observed in subgroup C. Negative correlation between urinary osmolality and urinary FN was found (r=-0.426, p < 0.01). In children with VUR, serum FN increased with reflux grade, whereas its urinary level was elevated only in grade IV and V reflux with impaired urine concentration.