Continuous monitoring of blood oxygen tension was carried out during 45 hemodialyses sessions. The oxygen tension curves displayed different appearances. Some curves presented with a periodicity like sinus curves (type 1), while others (type 2) fluctuated with other appearances, or displayed straight lines. Blood pressure variations during dialysis were significantly greater during dialyses displaying type 1 curves than during those displaying type 2 curves (p values < 0.02). Due to a greater descend of S-urea during dialyses with type 1 curves than during dialyses with type 2 curves (p < 0.05) it is hypothesized, that the intensity of the dialysis treatment could be of some importance to the observed fluctuations in oxygen tension.