The late B. Tersteegen devised a clever way to combine the advantages of a closed tank hemodialysis system with the efficacy and bacteriological safety of a single-pass system. The Teerstegen equipment is now marketed as the GENIUS ® hemodialysis system. For each treatment, fresh dialysis fluid is prepared according to the physician’s prescription by mixing sterile ingredients (electrolytes and glucose) with preheated ultrapure water. The total amount of dialysis fluid is put into a thermally insulated glass tank (volume 75 l) of the hemodialysis machine. The filling and emptying process is completely automated. An UV radiator is used for desinfection. Due to a consequent hygienic concept, the system operates with an almost sterile and usually pyrogen-free dialysis fluid. During treatment, fresh dialysis fluid is taken from the top of the system, and the used dialysate is returned to the bottom. There is a sharp interface between the fresh and used dialysis fluids because of a small difference in temperature (1°C). True volumetric ultrafiltration control is simply achieved; ultrafiltration rates between 100 and 1,000 ml/h can be selected as considered appropriate by the physician. Microbiological examination: We cultured more than 2,000 dialysate specimens to examine bacterial contamination, and found either no bacterial growth at all (in the vast majority of the cases) or less than 1 CFU/ml dialysate. Clinical experience: We have utilized the GENIUS ® system since 1994. Meanwhile, more than 40,000 treatments have been performed in our center. Biochemical results of the first 19 stable hemodialysis patients (mean age 66 years, range 45–82), who had been treated with conventional hemodialysis systems for at least 6 months (range 6–157) before changing to GENIUS ® , were evaluated. We observed an increase of mean serum albumin concentration from 4.1 (±0.4 SD) g/dl to 4.8 (±0.3) g/dl (p ® system in comparison to conventional hemodialysis machines. Conclusion: The GENIUS ® hemodialysis system permits an individualized therapy of high quality; most patients prefer this system to conventional hemodialysis machines. Serum albumin levels increased significantly from normal to high normal values after change from conventional hemodialysis machines to GENIUS ® , probably due to less catabolic stress during the hemodialysis sessions.