The possibility of sepsis or pyrogenic reaction increases in hemodialysis patients as coliform or other gram-negative counts in the dialysis water reach levels ≥ 1000 colonies/ml. After the introduction of Cobe Century II dialysis machines (Cobe, Lakewood, Colorado), routine testing of dialysis water samples revealed gram-negative bacteria in concentrations up to 45,000 colonies/ml. An epidemiologic investigation was performed to determine the source of the bacterial contamination. The amounts of disinfectant solution were increased, but bacterial counts remained unacceptably high. A study of the fluid path within the machines revealed that part of that path was not reached at all by the disinfectant. In addition, the disinfectant was automatically and improperly diluted. Therefore, increased bacterial counts could not be reduced satisfactorily by using the manufacturer's disinfection guidelines.