Fluoroquinolone prophylaxis in patients with profound neutropenia may be useful for preventing gram-negative bacterial infection, but it is ineffective against gram-positive bacterial infections in the bloodstream, particularly those caused by streptococci and coagulase-negative staphylococci, which appear to have emerged as significant causes of morbidity, decreased treatment efficacy, and the increased costs of empiric antimicrobial therapy. In a prospective, randomized, open trial, we evaluated the efficacy and safety of oral roxithromycin (150 mg twice daily) as additional antibacterial prophylaxis in 131 adult patients with acute leukemia and bone marrow transplant recipients receiving oral ofloxacin. In comparison with patients given ofloxacin alone, fewer patients receiving ofloxacin plus roxithromycin developed bacteremia caused by viridans group streptococci (incidence, 9 versus 0%; P = 0.03), while the incidence of bacteremia caused by other organisms, the incidence of febrile episodes from any cause, the risk of infection-associated complications (including prolonged or secondary fever, pneumonia, septic shock, need for mechanical ventilation, and/or infection-related death), and antimicrobial usage for therapy were comparable between both groups. Adverse events possibly related to the study drugs were slightly more common among the patients receiving the combination treatment (P = 0.05). Although effective for the prevention of streptococcal bacteremia, the addition of roxithromycin to a fluoroquinolone should not be used routinely as a prophylactic regimen in patients with profound neutropenia, but it might be considered and may be useful for cancer patients with a particularly high risk of streptococcal infection and related complications.