Background. Within-household transmission of Escherichia coli may promote urinary tract infection (UTI) but is poorly understood. Methods. Fecal samples from 228 individuals (152 humans [5 with acute UTI] and 76 pets) in 63 households were extensively processed for unique E. coli clones, as defined by random-amplified polymorphic DNA analysis and pulsed-field gel electrophoresis. Patterns of strain sharing (presence of a clone in multiple individuals) were assessed. Results. Of 335 E. coli clones, 90 (27%) were recovered from multiple hosts (up to 11 per clone). Withinhousehold strain sharing (1) involved 68% of households, including 3 of 5 households in which a member had a UTI; (2) was more frequent than across-household strain sharing (27% vs. 0.8% of potential sharing pairs; P < .001); (3) increased with household size (r2 = 0.93; P < .001); and (4) varied by host-pair type (pet-pet, 58%; human-human, 31%; human-pet, 17%). Sex partners shared strains more commonly than did other adults (31% vs. 7% of pairs; P = .08) but accounted for only 12% of within-household strain sharing. Conclusions. Within-household sharing of E. coli, including in households in which a member has a UTI, is common and can involve any combination of humans and pets. Identification of the underlying mechanism(s) could lead to novel preventive measures against UTI.