Antibiotics are substances commonly consumed by the population under medical prescriptions. These compounds may reach the environment through human excretion as they are partially eliminated in WWTPs. The occurrence of antibiotics in the aquatic environment is therefore of particular concern as they can present a risk for both ecosystems and human health. Furthermore, they have the potential to foster bacterial resistance. One source of antibiotics in the environment can be linked with hospitals activities. In this study, antibiotics were measured in the effluent of the main building (650 beds) of the biggest hospital of Lausanne city (CHUV, 1200 beds), Switzerland. In parallel, the entry and the outlet of the WWTP of the city as well as the receiving waters (lake Geneva) were also investigated, collecting samples of wastewater or lake water. Nine antibiotics (azithromycin, ciprofloxacin, clarithromycin, clindamycin, metronidazole, norfloxacin, ofloxacin, trimethoprim , sulfamethoxazole) were analyzed with SPE extraction followed by LC-MS/MS analysis. Results showed very high concentrations in the range xxx µg/L at the outlet of the hospital that may represent a risk of bacterial resistance induction. Data gained in this hospital was extrapolated to all clinics and medical centers of the city (representing 3540 beds in total ) and compared with antibiotics measurements at the entry of the WWTP. The total antibiotic mass at the entry of the WWTP of all hospitals and medical centers represent a small fraction of the total measured amount of antibiotics (< 5%) except for ciprofloxacin (15,1%) and sulfamethoxazole (23.6%). Similar results were obtained in a study at the hospital site in Baden. For some antibiotics (azithromycin, metronidazole, norfloxacin), concentrations at the entry of the WWTP were higher in comparison with hospital concentrations. This demonstrated an important consumption of antibiotics by the population outside of medical centers. After treatment in the WWTP, concentrations of antibiotics were reduced but were still bigger than PNEC values for clarithromicyn and sulfamethoxazole. Finally, after dilution in Geneva Lake, the measured concentrations are below effects values. However, the risk of multiresistant bacterial discharged from hospitals is another issue that needs consideration.