Background. Infection results in considerable morbidity and mortality in haemodialysis patients. Diagnosis of infection can be difficult because currently applied laboratory parameters may be non-specifically altered due to uraemia or haemodialysis (HD). This study investigated the diagnostic value and kinetics of serum procalcitonin (PCT). a low-molecular-weight protein, in patients receiving intermittent HD. Methods. Sixty-eight patients receiving intermittent HE for end-stage renal disease (n = 48) or acute renal failure (n = 20) were prospectively studied, 47 treated with high-flux and 21 with low-flux membranes. Of 36 patients with severe infections or sepsis, 27 were treated with high-flux and nine with low-flux membranes. WBC, serum PCT and C-reactive protein (CRP) concentrations were measured immediately before HD, and PCT repeatedly during the following 48 h. Results. When determined immediately before IIB, PCT demonstrated a sensitivity of 89%,, a specificity of 81%, and positive and negative predictive values of 84 and 87%, indicating severs infection or sepsis. These levels were higher than the respective values for CRP (89, 48, 68 and 78%) and WBC (58. 75, 71 and 59%). After 4 h of HD with high-flux membranes, PCT decreased significantly to 83 +/- 25% and did not return to predialysis concentrations before 48 h. This decrease in serum PCT resulted in markedly reduced sensitivity (65%) and negative predictive value (54%). In contrast, no marked change in PCF concentration occurred during or after HD with low-flux membranes. Conclusion. Serum PCT is an accurate indicator of severe infection and sepsis in patients receiving intermittent HD. High-flux, membranes substantially decrease PCT. When utilizing high flux membranes, serum PCT concentrations should be determined prior to the start of I-IB.