In various pathological conditions elevated serum sialic acid level, as the result of increased metabolism of glycoproteins and glycolipids, is observed. The study aimed at the evaluation of sialic acid concentration in serum of children on continuous ambulatory peritoneal dialysis (CAPD) and maintenance haemodialysis (HD). Examination was performed in 27 end-stage renal disease (ESRD) children including 11 on CAPD, 16 on HD treatment. Ten healthy children served as the control group (K). In CAPD group assessment was carried out during the routine monthly check up, in HD group--before (HD-1) and after (HD-2) dialysis session. Sialic acid concentration was determined using method of Shamberger. In CAPD children we obtained significantly increased serum sialic acid concentration comparing to controls and HD children. Increased serum sialic acid concentration was also found in HD children comparing to controls. There was no significant difference between the sialic acid concentrations before and after dialysis session. Analysis of correlation revealed positive correlation of sialic acid concentration with haemoglobin concentration and hematocrit in CAPD group. In children on HD treatment we showed positive correlation of sialic acid level with renal replacement therapy duration and creatinine concentration after HD. Elevated serum sialic acid concentration in dialysed children could be the result of non-specific organism response, characterised by tissue and organ damage, towards the materials of extracorporeal circulation (HD) or the presence of dialysis solution in peritoneal cavity (CAPD).