Reuse of hemodialyzers has continued to increase dramatically. In 1983, 52% of the hemodialysis centers were reusing hemodialyzers. Use of hollow fiber hemodialyzers has paralleled this increase in hemodialyzer reuse with 63% of the hemodialysis centers reporting exclusive use of this type of hemodialyzer in 1983. Although reuse of hemodialyzers has not been associated with increased incidence of hepatitis B infection or pyrogenic reactions, an outbreak of nontuberculous mycobacteria in 27 chronic hemodialysis patients was probably associated with water used to reprocess dialyzers. Nontuberculous mycobacteria were isolated from the water in 83% of 115 hemodialysis centers surveyed across the United States and could constitute a potential infection risk because of the organisms' greater germicide resistance than most other naturally occurring water bacteria. Two percent formaldehyde is not an effective germicide for high level disinfection of hemodialyzers. Reprocessed hemodialyzers should be disinfected with 4% formaldehyde or an equivalent disinfectant.