Objective: To study the epidemiology of resistant enterococci colonizing oncology patients. Setting: Oncology outpatient and inpatient ward of a Canadian tertiary care pediatric center. Design: Prospective observational trial. Patients: Patients < 18 years of age undergoing chemotherapy for malignancy. Methods: Patients submitted stool samples or had rectal swabs obtained once monthly over a 12 month period, either during clinic visits or prior to antibiotics if admitted for febrile neutropenia. Enterococcus was identified by routine biochemical testing and resistance to ampicillin (ARE), gentamicin (GRE) and vancomycin(VRE) were determined. Ciprofloxacin resistant (CRE) strains were identified and underwent pulsed field gel electrophoresis (PFGE) to determine if cross contamination was occuring. Results: 128 stool samples were obtained from 30 children. The mean number of samples submitted from all children were 4.3±3.7 (range 0 to 12). 5 children were identified with resistant enterococci: 3 with ARE, all isolates E. faecium; 3 children were identified with VRE, 2 isolates E.gallinarum and 1 E. casseloflavus;and 1 case of high level GRE was noted (E. faecalis). 2 children had more than 1 type of resistance. Once identified, the resistant strain tended to persist: average 3.7 months (range 1-6 months) and was intermittent in 1 case with VRE being found before and after 4 negative cultures. Children with resistant strains were not different in terms of demographics (mean age 4.5±2.4 years, 20% male, 40% solid tumour) compared to those without(mean age 5.0±2.1 years, 72% male, 28% solid tumour)(all p>0.05). 21/22 resistant isolates were collected in the ambulatory unit. One 5.5 yr old girl with ALL who was Philidelphia marker positive had no resistance documented on 2 swabs (last one 2 weeks before) prior to transfer to a large tertiary care center for bone marrow transplantation. Upon transfer back, both high level GRE and ARE were found. PFGE was performed on 13/16 patients carrying CRE; same strains were shared by 3 pairs. Conclusions: Resistant enterococci are common in children with cancer attending the ambulatory clinic. Once colonization occurs, it tends to persist. Hospitalization at large tertiary care centers may be a risk factor for acquisition. Since spread of enterococci does occur between these patients, good surveillance for ARE, GRE and VRE and adherence to infection control guidelines is necessary.