Haemodialysis-induced hypotension is still a severe complication in spite of all the progress in haemodialysis treatment. Because of its multifactorial causes, haemodialysis-induced hypotension cannot be reliably prevented by conventional ultrafiltration and sodium profiling in open-loop systems, as they are unable to adapt themselves to actual decreases in blood pressure.A blood-pressure-guided closed-loop system, for prevention of haemodialysis-induced hypotension by biofeedback-driven computer control of both ultrafiltration and saline infusion was clinically tested in 237 treatments of seven patients prone to hypotension. As medical knowledge on multifactorial causes of hypotension is characterized by a lack in deterministic knowledge, fuzzy logic and linguistic variables were used to involve clinical experience on hypotension phenomena in terms of fuzzy knowledge. Biofeedback control is based on frequent measurements of blood pressure at 5 min intervals. Blood pressure behaviour is described by linguistic variables and fuzzy sets. Adaptive rule bases were used for the simultaneous fuzzy control of both the ultrafiltration and infusion of hypertonic saline (20% NaCl). Proper adaptation of control features to patient's conditions was provided by the critical borderline pressure, which was set by the physician individually at the beginning of each treatment. During the initial and medium phases of the sessions, ultrafiltration rates up to 150% of the average rates were applied as long as decreases in blood pressure could be compensated by saline infusion. The surplus of ultrafiltrate volume was used for blood pressure stabilization in the final phase in most instances by low ultrafiltration rates.The advantages of biofeedback-controlled haemodialysis were demonstrated by both decreasing the frequency of hypotonic episodes and by increasing or maintaining constant levels of systolic blood pressure during the final phase in 88% of treatments. As saline infusion was applied mainly in the initial and medium phases, blood sodium levels were not significantly higher at the end of the sessions, and interdialytic weight gain was not elevated.The application of fuzzy logic in the blood-pressure-guided biofeedback control of ultrafiltration and sodium infusion during haemodialysis is able to minimize haemodialysis-induced hypotension.