Background Extended-spectrum β-lactamase-producing Escherichia coli has become ubiquitous and has been reported in diverse ecosystems. We evaluated the potential impact of post-acute and long-term healthcare activities on the environment by quantifying ESBL-producing Enterobacteriaceae in wastewaters of a French geriatric hospital. Methods We collected wastewater specimens representative of one-day efflux immediately before the connection with the municipal sewer pipe. The sample was processed following two different methods: dilution-filtration method and concentration method and was screened for ESBL-producing Enterobacteriaceae using selective media. ESBL E. coli strains were quantified, screened for ESBL genes and compared with ESBL strains isolated from patients present in the building at the time of wastewater collection, using molecular methods. Results Six distinct environmental ESBL E. coli clusters were identified, two of them related to patient strains. The concentrations in hospital wastewater of these strains ranged from 2.5 × 104 to 106 UFC/L. Conclusions Our results demonstrate that the presence of ESBL E. coli patients leads to a dissemination of ESBL E. coli in the environment and highlights the need to improve excreta and wastewater policy in hospitals.