60 patients with 60 viridans streptococcal bacteraemic episodes (42 due to penicillin-sensitive and 18 due to penicillin-resistant viridans streptococci) were analysed in a population of 12,185 admissions and 1,380 bacteraemic episodes during a 7-year period in a National Cancer Institute. The incidence of viridans streptococci among bacteraemias decreased from 11.5% in 1989 to 2.5% in 1995 after penicillin was introduced for prophylaxis of febrile neutropenia in acute leukaemia in 1993. However, the proportion of penicillin-resistant viridans streptococcal bacteraemias increased from 0 in 1989 and 1990 before any prophylaxis was given, to 12.9-16.7% after quinolones were used for prophylaxis in 1991 and 1992, and to 44.4-81.8% in 1993-1995 after penicillin was added to the quinolones. Mortality rate was higher in the subgroup of penicillin-resistant viridans streptococcal bacteraemias (p < 0.05). Statistically significant risk factors in patients with penicillin-resistant (compared with penicillin-sensitive) viridans streptococcal bacteraemia were: acute leukaemia (p < 0.03), high doses of cytarabine (p < 0.05), mucocutaneous lesions (p < 0.004), breakthrough bacteraemia during prophylaxis with ofloxacine plus penicillin (p < 0.001). Multiple logistic regression analysis showed that only acute leukaemia (OR 2.05, CI 0.85-1.85, p < 0.00452) and penicillin-resistance (OR 0.71, CI 0.103-4.887, p < 0.0209) were significant independent predictors of inferior outcome. Breakthrough bacteraemia during empiric therapy with vancomycine occurred in 5 of 116 patients treated with vancomycine, and during therapy with ampicillin plus gentamicin in 6 patients of 18 treated.