One hundred twenty three breakthrough bacteraemias (BB) during 5 years in a National Cancer Institute, among 9986 admissions and 979 bacteraemic episodes were analysed. 123 BB were caused by 323 microbes, only 116 were resistant (31.5%) to currently administered antimicrobials. Sixty seven of 123 bacteraemic episodes were catheter associated confirmed by isolation of the same organisms from the blood and catheter tip. 77/123 BE were polymicrobial. The most frequently isolated strains were coagulase negative staphylococci (30.5%), Corynebacteria (10%), Ps. aeruginosa (10%), Str. faecalis (9%) and Viridans streptococci (8.5%). Gram-positive aerobes accounted for two-thirds of all organisms isolated during breakthrough bacteraemic and fungaemic episodes. Mixed polymicrobial breakthrough bacteraemic and fungaemic episodes were more frequently associated with vascular catheter insertion and neutropenia, and had a less favourable outcome in comparison to monomicrobial infections. The relapse was associated more frequently with catheter related bacteraemic and fungaemic episodes, but the overall mortality rate was similar independently from catheter insertion. Breakthrough bacteraemic and fungaemic episodes were associated more frequently with acute leukaemia. Polymicrobial breakthrough bacteraemic and fungaemic episodes were associated more frequently in neutropenic episodes and in venous catheters. Regarding the outcome, an extraction of the catheter with no dependence on variable and modification of antimicrobial therapy were essential for the improvement in the prognosis. (Tab. 5, Ref. 20.).