Summary:The efficacy of ciprofloxacin as antibacterial prophylaxis for allogeneic bone marrow transplantation has been well documented, and it virtually eliminated bacteremias caused by gram-negative pathogens in early reports. Ciprofloxacin was therefore incorporated into the prophylactic antibiotic regimen during allogeneic bone marrow or peripheral blood stem cell transplantation at Veterans General Hospital, Kaohsiung from February 1997. In 12 consecutive patients receiving allogeneic bone marrow or peripheral blood stem cell transplantation, ciprofloxacin-resistant Escherichia coli bacteremia developed in three (25%). In addition to our data, increasing evidence suggests that the widespread use of a fluoroquinolone is associated with the emergence of resistant isolates as well as documented infections caused by these resistant strains. The incidence of Escherichia coli bacteremia in our transplant patients was 25%, which was similar to that in patients not receiving preventive therapy or in those receiving trimethoprim-sulfamethoxazole prophylaxis. The prophylactic efficacy of ciprofloxacin in allogeneic bone marrow transplant or peripheral blood stem cell transplant recipients should therefore be reassessed.