Infections account for considerable morbidity and mortality in patients requiring haemodialysis (HD). Procalcitonin (PCT)-a low molecular weight protein of 13 kDa-helps one to distinguish viral from bacterial infections and to evaluate the severity of bacterial infections. We investigated (1) PCT baseline levels in eight children undergoing chronic HD with high-flux membranes and (2) changes in the serum levels of PCT, C-reactive protein (CRP) and beta-2-microglobulin (beta2-MG)-a peptide with biochemical characteristics similar to those of PCT-before and after haemodialysis sessions. Blood sampling was performed three times in the mid-week session. Serum PCT of the seven uninfected children before HD sessions was increased (0.75+/-0.07 ng/ml), whereas CRP levels were normal. PCT after dialysis decreased significantly by 40% (P<0.0001) compared with initial values, whereas CRP levels before and after HD were not different. beta2-MG decreased by 70%, probably due to different biochemical properties of both proteins. PCT serum levels 15 min and 60 min after the HD session remained unchanged in comparison with those at the end of the HD session, suggesting accumulation of PCT between HD sessions rather than HD-induced production to be responsible for the increased baseline PCT serum levels. We concluded that CRP serum levels were not affected by HD in our group. Moderately elevated baseline PCT serum levels that are presumably due to reduced renal clearance and uraemia and dialysis-ability of PCT should be taken into consideration. However, increase of serum PCT in patients with severe bacterial infections is generally massive (10-fold to 1,000-fold), suggesting a low risk for false negative results in such cases.