Objective. There is growing concern about bacterial resistance to antimicrobials. The majority of antibiotics used are only partially metabolized after administration, and are released via patient excreta into the municipal sewage system. Data on the use of antibiotics and their emission into hospital effluent are not available. Methods. Antibiotic consumption in Germany was calculated on the basis of five hospitals of varying size and medical service spectrum and on prescriptions issued by medical practitioners. The predicted environmental concentration (PEC) was calculated for hospital effluent and for municipal sewage. The PECs were compared both with published minimum inhibitory concentrations (MIC ₅₀) for sensitive pathogenic bacteria and with the predicted no-effect concentrations (PNECs). Results. The amount of antibiotics emitted into hospital effluent may reach and exceed the MIC ₅₀ of susceptible pathogenic bacteria. The PEC/PNEC ratio is highest for hospital effluent (in some cases 10-20 times the MIC ₅₀) and frequently >1 for municipal sewage. PECs are high enough for some compounds to have a PEC/PNEC ratio >1 even in surface water. Conclusion. The volume of antibiotics used in hospitals and private households and released into effluent and municipal sewage indicates a selection pressure on bacteria. Steps should be taken to reduce the risk by proper handling of antibiotics and their residues both in hospitals and by private users.