We prospectively studied 485 episodes of bacteremia in neutropenic patients with cancer. Viridans streptococci caused a total of 88 episodes (18%). Ten (11%) of these 88 cases were associated with serious complications: acute respiratory distress syndrome (ARDS) plus septic shock (5 cases), ARDS (3), and septic shock (2). Streptococcus mitis was the species most frequently isolated (7 of 10 episodes). Four viridans streptococci showed a diminished susceptibility to penicillin (MICs ranged from 0.25 to 4 microg/mL), and 5 strains were resistant to ceftazidime (MICs ranged from 2 to >32 microg/mL). Patients with viridans streptococcal bacteremia (VSB) who developed serious complications were compared with patients with VSB without complications. Severe oral mucositis (70% vs. 32.5%, respectively; P=.036), high-dose chemotherapy with cyclophosphamide (60% vs. 25%, respectively; P=.043), and allogeneic bone marrow transplantation (40% vs. 10%, respectively; P=.040) were the only variables found to be significantly associated with the development of complications. Neither a specific species of viridans streptococci nor resistance to penicillin was associated with the occurrence of complications. The mortality rate was higher in case patients than in control patients (80% vs. 17.5%, respectively; P<.001). Serious complications associated with VSB occur mainly in patients receiving high-dose chemotherapy with cyclophosphamide before allogeneic bone marrow transplantation who develop severe oral mucositis; these complications are associated with a high mortality rate.