In 61 consecutive myeloablative allogeneic hematopoietic stem cell transplant recipients, viridans streptococci were the most common cause of bacteremia, accounting for 19 of 31 (61 percent) cases of bacteremia occurring during the period of post- transplant neutropenia. Seven patients had more than one species of viridans streptococcus in the blood. The most common species was Streptococcus mitis. Most of the streptococcal isolates were resistant to norfloxacin, an antibiotic used for prophylaxis. A comparison of these 19 patients with a group of 23 transplant recipients with fever and neutropenia, but no identified focus of infection, revealed that patients with viridans streptococcal bacteremia were more likely to have severe intraoral pathology. Results of this analysis suggest that poor underlying dental health and the use of norfloxacin for prophylaxis predispose this patient population to viridans streptococcal bacteremia.