The impact of the combination therapy fludarabine plus cyclophosphamide (FC) in comparison with fludarabine alone regarding the incidence and severity of infections among previously untreated patients with chronic ;lymphocytic leukaemia (CLL) was evaluated within a multicentre phase III study. A total of 375 patients, up to 65 years old, were randomised between fludarabine or FC for first line therapy. No routine anti-infective prophylaxis was provided. A total of 196 infectious episodes, including 33 severe infections, were documented. In the fludarabine arm, 32.9% of the patients developed an infectious complication compared with 39.9% in the FC arm (P = 0.2). No difference was observed in the rate of severe infections (Common Toxicity Crtieria grades III and IV) between both treatment arms. Dose reductions were performed more frequently in FC-treated patients. Granulocyte colony-stimulating factor (G-CSF) was administered due to leucopenia in 5% of all patients. A multivariate regression model identified only elevated thymidine kinase, but not the treatment arm, as a statistically independent risk factor for infections. In summary, FC was not associated with a higher rate of infections compared with fludarabine alone. No routine antibiotic or virostatic prophylaxis, or pre-emptive treatment with G-CSF, is necessary in first line therapy with fludarabine-based regimens in younger patients with CLL, if adequate dose reduction is performed. The combination therapy FC is not associated with a higher rate of infections compared with fludarabine alone. No routine antibiotic or virostatic prophylaxis as well as pre-emptive treatment with G-CSF is necessary in first line therapy with fludarabine-based regimen in younger patients with CLL, if adequate dose reductions due to cytopenia or previous infections are performed.