Between April and November 1982, 27 of 140 patients in a hemodialysis center in Louisiana were infected with rapidly growing mycobacteria; 14 had bacteremia alone, 3 had soft-tissue infections, 1 had an access-graft infection, and 9 had widely disseminated disease. Of 26 identified isolates, 25 were Mycobacterium chelonei ssp. abscessus, and one was an M. chelonei-like organism. One factor common to all patients was exposure to processed hemodialyzers (artificial kidneys). Environmental sampling of the water-treatment system showed widespread contamination with nontuberculous mycobacteria, which were also recovered from the patient's side (blood compartment) of five of 31 hemodialyzers that had been processed and were ready for use. The formaldehyde concentration was less than 2% in two of three such contaminated dialyzers tested. We hypothesize that patients became infected when their blood circulated through processed dialyzers that contained viable rapidly growing mycobacteria. This outbreak demonstrates that hemodialysis patients may be at risk for developing infections with rapidly growing mycobacteria and that such infections may go unrecognized when routine culture methods are used. It also emphasizes the importance of using effective procedures to disinfect dialyzers in hemodialysis centers.