The extent to which plasma potassium determines potassium removal in hemodialysis was examined in 8 end-stage renal disease patients during 51 treaments. Dialyzers, treatment time, blood and dialysate flow were held constant. Dialysate composition was also uniform except that 24 treatments utilized glucose-free dialysate and the remaining 27 a 200 mg/dl glucose bath. At either level of dialysate glucose, approximately 40% of the potassium removal during dialysis could not be accounted for by plasma-dialysate potassium gradient, body weight or serum carbon dioxide content, although glucose-free dialysate tended to increase potassium removal by a mean of 28%. The large, unexplained variability in potassium removal suggests that therapeutic manipulation of potassium flux across cell membranes may improve the management of potassium balance in hemodialyzed patients.