The authors discuss the morbidity associated with viridans streptococcus bacteremia, and its implications on the choice of antibiotics used as prophylaxis and treatment. They retrospectively studied the case notes of 38 children who were being treated for various malignant conditions in their unit and developed 40 episodes of bacteremia with viridans streptococci between October 1995 and January 1999. Viridans streptococci were the third commonest blood culture isolate during this period, after coagulase-negative staphylococci and Staphylococcus aureus. The majority of the isolates were Streptococcus mitis (55%). Others were S. sanguis (25%), S. oralis (12.5%), S. salivarius (5%), and S. acidominimus (2.5%). Twenty-five percent of the patients had been treated with regimens that included cytosine arabinoside, 60% were receiving prophylactic co-trimoxazole, and 87.5% were neutropenic. Thirty percent of patients had abnormal chest X-rays, and 15% were hypotensive; 2 patients required admission to the intensive care unit. Initial antibiotic therapy was changed because of failure of clinical response in 60% of cases, despite the infecting organism being sensitive in vitro. This study confirms the importance of viridans streptococci as a cause of bacteremia in pediatric hematology and oncology patients, leading to significant morbidity. Further work is required to establish the optimal treatment for viridans streptococcus bacteremia.