A reverse osmosis plant for preparation of dialysis water is described. With a total cellulose acetate membrane area of 1.8 m<sup>2</sup> and a high flow rate of the water across the membrane a filtrate production of about 500 ml/min was obtained at a mechanical pressure of about 35 atmospheres. This is adequate for long-term dialysis of one patient. The capacity can be increased at will by extending the membrane area or by serial arrangement of 2 or more columns. After 6 h of continous operation the filtrate had a calcium concentration below 0.6 mEq/l (on an average 0.3 mEq/l), a sodium concentration below 0.5 mEq/l, and a conductivity of 20–60 //mho. The filtrate was sterile and pyrogene-free. The apparatus is easy to work and proved to be safe and cheap to run.