A multiresistant E. cloacae strain spread during a six month period in a paediatric oncology ward amongst nine children, who had different tumors and malformations. Three children who had shared a room were especially affected. E. cloacae was isolated 122 times from the children with tumors and five times from their environment. Specimens from which the bacteria were isolated, included blood cultures, catheter tips, wound swabs, drains, skin and mucous membranes from most parts of the body. The majority of the E. cloacae strains were resistant to ampicillin, mezlocillin, piperacillin, azlocillin, doxycycline and cephalosporins of the second and third generation and sensitive to imipenem, aminoglycosides and quinolones. The antimicrobial resistance patterns of the E. cloacae strains from the paediatric oncology ward were compared to those isolated from other wards in the hospital. E. cloacae isolates from the intensive care unit had a reduced sensitivity to beta-lactam antibiotics, whereas the isolates from the other wards were, with the exception of ampicillin, sensitive to beta-lactam antibiotics. The analysis of the E. cloacae strains from the paediatric oncology ward revealed the same antimicrobial resistance pattern, bacteriocin type, RFLP-type and an identical enzyme and whole cell profile. Isolates from other wards showed considerably deviating patterns. The systematic registration and isolation of patients, colonized or infected with multiresistant E. cloacae strains, together with infection control methods, lead to a significant reduction in infections.