We investigated 18 pyrogenic reactions (PRs) that occurred between July 1 and 13,1987, in 16 patients receiving long-term hemodialysis at one dialysis center in Illinois. We defined a case of PR as the onset of chills or fever (oral temperature, ≥37.8°C) in a patient who was afebrile and had no signs or symptoms of infection before a dialysis treatment. Pyrogenic reactions to dialysis sessions during the epidemic period (July 1 through 14) were associated with the use of a reused dialyzer (risk ratio, 6.2; 95% confidence interval, 0.8 to 45). The water used to rinse the dialyzers after use and to dilute the germicide for disinfecting the dialyzers contained high concentrations of endotoxins (>6 ng/mL) and bacteria (>104 colony forming units/mL). After reuse of dialyzers was discontinued at the center, the PR rate fell to pre-epidemic levels. We concluded that PRs were associated with reuse of dialyzers, probably contaminated with endotoxins. Active surveillance for PRs among patients undergoing long-term hemodialysis should be routine at all dialysis centers; an increase in PRs should prompt timely review of water treatment and dialyzer reprocessing systems.