Based on findings in 2 fluoride-toxic patients, it was suspected that hyperkalemia played a clinically important role in the etiology of sudden death from fluoride poisoning. Using fluoridated human erythrocytes as an in vitro model, it was confirmed that fluoride produced a marked potassium efflux from intact cells. Further, neither glucose and insulin in pharmacologic doses, nor various buffers could halt the efflux by shifting the potassium intracellularly. If these results can be extrapolated to the clinical situation, removal of potassium and fluoride via exchange resins or dialysis remains the only reasonable approach to this life threatening problem. Aside from sudden hyperkalemia and hypocalcemia, no serologic marker for fluoride toxicity has been identified. A high degree of clinical suspicion is therefore essential to the diagnosis.