These data indicate that a patient maintained by hemodialysis in a community using fluoridated water may be exposed to a fluoride concentration higher than that present in tap water if the deionizer is allowed to become exhausted while the patient is being dialyzed. The concentration reached 520 uM in the laboratory deionizer and 230 M in the patient’s model. Since the concentration of fluoride in blood returning from the dialyzer is about one-half to two-thirds that in the dialysate, the concentration could reach 150 uM in human serum. This would probably not be fatal inasmuch as concentrations of 500 to 1,400 uM are necessary to cause death of the rat or rabbit [11, 12]. Repeated exposures to excessive fluoride concentrations, however, could lead to saturation of the skeleton with fluoride and sustained elevation of serum fluoride to concentrations of more than 30 uM. Under these circumstances, loss of weight and disturbance of bone mineralization or other toxic effects could occur. The excessive amounts of osteoid seen in the bone biopsy specimen and the decrease in osteomalacia subsequent to correcting the deionizer operation are consistent with a fluoride effect. However, it is not necessary to prove that our patient was harmed by excessive concentrations of fluoride to make the point that unwarranted risks are taken when fluoride is allowed to accumulate in the deionizer and to elute during dialysis, and to suggest that there be a systematic check to guard against the use of a deionizer beyond its capacity.