Previous reports have suggested an increasing incidence of highly fluoroquinolone-resistant Escherichia coli causing bacteremia among cancer patients on prophylactic therapy. We used genotyping by pulsed-field gel electrophoresis of chromosomal DNA digests and random amplified polymorphic DNA fingerprinting to study clonal relationships among such isolates obtained at 10 cancer centers located across Europe and the Middle East. Analysis by both methods indicated that isolates from different centers were genotypically unrelated to each other. There were five centers from which more than one individual patient isolate was available, and most demonstrated significant within-center genetic diversity of strains. Strains shared among patients could be identified at two centers. At the center with the largest number of bloodstream isolates from cancer patients available, fluoroquinolone-resistant control isolates from surgical patients and fluoroquinolone-susceptible control isolates from patients admitted to medical services during the same time period were unrelated to resistant cancer patient isolates and to each other as well. A substantial number of fluoroquinolone-resistant isolates (19 of 58) were nontypeable by pulsed-field gel electrophoresis. Fluoroquinolone resistance was commonly associated with multiple antibiotic resistance to chemically unrelated antibacterial agents irrespective of the origin of the isolates.