Few studies have examined the specific incidence of infections after autologous bone marrow transplantation (BMT) or peripheral stem cell transplantation (PSCT) for treatment of breast cancer. We reviewed the medical records of 127 consecutive patients who underwent autologous BMT or PSCT for breast cancer at the University of Pennsylvania Medical Center from 1 May 1991 through 31 March 1995 and through 1 year of follow-up. The mean duration of neutropenia after transplantation was 10 days. Initial infections included catheter-site cellulitis (in 20 patients [16%]), bacteremia (17 [13%]), Clostridium difficile colitis (13 [10%]), and urinary tract infection (in 10 [8%]); there was only 1 documented invasive fungal infection (1% of patients). The mortality from infection was 2%. Infections during the 1 year follow-up included upper respiratory infections (11 patients [10%]) and dermatomal zoster (9 [8%]); neither was significantly associated with death. This group of patients who underwent BMT or PSCT for breast cancer had a low rate of infectious morbidity and mortality. Viral and fungal infections were rare despite inconsistent prophylaxis.