A small cluster of waterborne septicemias with Pseudomonas species occurred in a dialysis unit despite regular control of bacterial contents of tap water and dialysate conforming to preset standards. Pseudomonas aeruginosa, P. maltophilia, and/or P. vesicularis were found in the blood cultures of three patients in whom four pyrogenic reactions developed. The cluster occurred only in patients treated with formaldehyde-reused dialyzers. Pseudomonads were also cultured from the effluent of two dialyzers reprocessed with formaldehyde and not yet used; these two dialyzers had extremely low formaldehyde concentrations. The tap water used for dialyzer rinsing in the reuse procedure contained only 220 colony forming units/ml pseudomonads. The problem appeared to be related to the inadequate mixing of the sterilant with the tap water used in the automated reprocessing device--in the absence of an alarm for this failure. After immediate discontinuation of the reuse procedure with this device (that had been in use for 6 years) no further septicemic episodes were registered. It is concluded that septicemias may occur despite a bacteriologic contamination level of water conforming to preset standards. Such episodes can be avoided only by total water decontamination.