A study of the blood chemistry parameters of 20 renal dialysis patients was undertaken when a local water district introduced chlorine dioxide (CIO2) as a disinfectant at the filtration plant headworks for 12 months without informing the renal dialysis clinic in the area of this potentially adverse change. Due to data limitations, including changes in clinical laboratories and lack of pre-exposure data for some patients, the analysis was focused on 17 patients for whom data was produced by the same clinical laboratory, for 3 months of pre-exposure and 1 month of exposure. Least-squares means of each parameter by CIO2 levels of 0.0 and 1.0 mg/L at the treatment plant were adjusted for age, sex, and creatinine. Water purification at the clinic included passing the water through granular activated carbon, filtration by 5-mu filters, and the use of reverse osmosis. Chlorination products measured at the clinic after this purification and prior to preparation of the dialysate consisted only of chlorite at the 0.02-0.08 mg/L level. No evidence of CIO2-induced anemia was found, nor were any other biologically significant responses observed. Study limitations include several potentially important hematologic parameters which were not measured, the small sample size, and three clinical laboratory changes.