Since 1977, our patients have undergone chronic HD with ultra-pure dialysate (UPD), defined as having endotoxin levels below 0.008 ng/ml and less than 1 bacteria/ml of dialysate. We evaluated the incidence of carpal tunnel syndrome (CTS) in three groups of patients. Group I (GI), 84 patients, dialysed for 6.1 +/- 3.2 years (mean +/- SD) with UPD only; Group II (GII), 39 patients, first dialysed for 3.7 +/- 2.3 years with non-UPD and afterwards for 8.4 +/- 2.1 years with UPD; Group III (G III), 103 patients treated for 6 +/- 5.9 years exclusively with non-UPD. All patients were dialysed with cuprophan or cellulose acetate membranes. Results, expressed by Kaplan-Meier actuarial survival curves as the percent of patients without CTS, show that CTS occurred significantly less in GI than in GIII. This may be due to less stimulation of monocytes resulting from the absence of bacteria, endotoxins and pyrogens in the dialysate, which would reduce the stimulation of cytokines release, interleukin 1 and 6, and tumor necrosis factor, known to stimulate beta 2 microglobulin synthesis.