The incidence rate (IR) of bloodstream infections (BI) and invasive mycoses (IM) during chemotherapy for paediatric acute lymphoblastic (ALL) or non-lymphoblastic leukaemias (AnLL) was evaluated for 153 BI and 22 IM diagnosed during 143,668 patient-days at risk from January 1988 to December 2000. IR, the number of episodes/100 days at risk, was 0.315 for AnLL and 0.092 for ALL (P < 0.001) with significant changes reflecting the intensity of anti-ALL chemotherapy. IR was 0.097 for first-line less intensive, 0.136 during first-line intensive, 0.261 during second-line therapy (P < 0.001), and 0.021 during maintenance. During intensive chemotherapy, the IR for BI was 0.134 in ALL with 0.087 for first-line less intensive therapy, 0.110 for first-line intensive, 0.230 for second-line intensive therapy (P < 0.001) and 0.274 in AnLL (P = 0.001). IR was 0.021 in ALL and 0.048 in AnLL (P = 0.034) for IM. In conclusion, there is a correlation between intensity of chemotherapy and rate of infections in paediatric acute leukaemias.