Aims: To detect ESBL (extended-spectrum b-lactamase)-producing Klebsiella pneumoniae present in the effluents and sludge of a hospital sewage treatment plant, evaluating the treatment plant’s potential to remove these microorganisms. Methods and Results: Twenty samples (crude sewage, UASB reactor effluent, filtered effluent and sludge) were collected in the period from May to December 2006, in order to analyse antimicrobial susceptibility and to check ESBL production, the disc-diffusion and the combined disc methods were used. Total and faecal coliform concentrations were also determined. ESBL-producing K. pneumoniae were detected in all samples analysed, representing 46Æ5% of the total strains isolated. Among the non-ESBL-producing strains, 26% were multiresistant and one strain resistant to eight of the nine antimicrobials tested was detected in the treated effluent. Conclusions: The hospital wastewater treatment plant did not show a satisfactory efficacy in removing pathogenic micro-organisms, allowing for the dissemination of multiresistant bacteria into the environment. Significance and Impact of the Study: The inefficacy of hospital wastewater treatment plants can result in routes of dissemination of multiresistant bacteria and their genes of resistance into the environment, thus contaminating water resources, and having serious negative impact on public health.