A significant proportion of hemodialysis patients have subnormal body temperature. Dialysis against cool dialysate has been frequently shown to reduce the incidence of symptomatic hypotension (SH), although only one of these reports included patient temperatures. Our hypothesis was that the response to cool or normal temperature dialysis could depend on a patient's baseline temperature. Of 128 patients in two hemodialysis units, 28 had a (mean of 5) baseline temperature less than 36 degrees C and 48 patients had a temperature higher than 36.5 degrees C. A crossover study was performed by dialyzing patients for 10 consecutive treatments with the same dialysate temperature, either 37 degrees C or 35 degrees C. All patients combined had a significant reduction in SH with 35 degrees C dialysate, 11.2% versus 5.5% with 37 degrees C dialysate (P = 0.001). The incidence of SH in euthermic patients was not affected by dialysate temperature. Hypothermic patients dialyzed against 37 degrees C dialysate had the highest incidence of SH, which decreased markedly with 35 degrees C dialysate (15.9% v 3.4%; P = 0.0001). There were no differences in age, duration of dialysis, gender, hemoglobin, urea, creatinine, or volume removed per dialysis between the two groups. In conclusion, subnormal temperature is common in dialysis patients but the etiology is unclear. The hemodynamic protective effect of cool dialysate only occurs in patients with subnormal temperatures. Only the subpopulation of patients with SH and low body temperature should be dialyzed against cool dialysate.