Aims: To investigate the bacterial diversity, antimicrobial resistance patterns and types of beta-lactamase genes in Gram-negative bacteria isolated from a hospital sewage treatment plant. Methods and Results: Between July and December 2008, we collected samples from influent, clarifier tank effluent and chlorine contact tank effluent from a sewage treatment plant service of a hospital located in the city of Rio de Janeiro, Brazil. Of the 221 isolates identified, 40% were characterized as extended-spectrum beta-lactamase (ESBL) producers. Nonpathogenic microorganisms and some pathogenic genera were quantified. The most common ESBL-producing isolates were Klebsiella pneumoniae, Enterobacter cloacae and Escherichia coli. The blaTEM, blaSHV and blaCTX-M genes were detected in 82, 48 and 67% of bacterial isolates, respectively. Conclusions: Results showed that hospital wastewater treatment plant is not suitable systems for the removal of all antibiotic-resistant micro-organisms present in hospital wastewaters. Significance and Impact of the Study: This study provides evidence that bacteria resistant to multiple antibiotics and their resistance genes that are usually present in the hospital can reach the environment, even after the use of hospital wastewater treatment plants.