Fil: Taha, Muhamed-Kheir. National Reference Centre for Meningococci. Institut Pasteur; Francia.Fil: Hedberg, Sara Thulin. Örebro University Hospital. National Reference Laboratory for Pathogenic Neisseria; Suiza.Fil: Szatanik, Marek. National Reference Centre for Meningococci. Institut Pasteur; Francia.Fil: Hong, Eva. National Reference Centre for Meningococci. Institut Pasteur; Francia.Fil: Ruckly, Corinne. National Reference Centre for Meningococci. Institut Pasteur; Francia.Fil: Abad, Raquel. Institute of Health Carlos III. Reference Laboratory for Neisserias; España.Fil: Bertrand, Sophie. Scientific Institute of Public Health. National Reference Centre for Neisseria meningitidis; Belgica.Fil: Carion, Francoise. Scientific Institute of Public Health. National Reference Centre for Neisseria meningitidis; Belgica.Fil: Claus, Heike. University of Würzburg. Institute for Hygiene and Microbiology; Alemania.Fil: Corso, Alejandra. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Departamento de Bacterilogía; Argentina.Fil: Enríquez, Rocío. Institute of Health Carlos III. Reference Laboratory for Neisserias; España.Fil: Heuberger, Sigrid. Austrian Agency for Health and Food Safety. National Reference Centre for Meningococci; Australia.Fil: Hryniewicz, Waleria. National Medicines Institute. National Reference Centre for Bacterial Meningitis; Polonia.Fil: Jolley, Keith A. University of Oxford. Department of Zoology; Reino Unido.Fil: Kriz, Paula. The National Institute of Public Health (NIPH). National Reference Laboratory for Meningococcal Infections; República Checa.Fil: Mollerach, Marta. Universidad de Buenos Aires. Cátedra de Microbiología Facultad de Farmacia y Bioquimíca; Argentina.Fil: Musilek, Martin. The National Institute of Public Health (NIPH). National Reference Laboratory for Meningococcal Infections; República Checa.Fil: Neri, Arianna. Instituto Superiore di Sanitá. Department of Infectious Parasitic and Immune-Mediated Diseases; Italia.Fil: Olcén, Per. Örebro University Hospital. National Reference Laboratory for Pathogenic Neisseria; Suiza.Fil: Pana, Marina. National Reference Center for Neisseria meningitidis. Cantacuzino Institute; Rumania.Fil: Skoczynska, Anna. National Medicines Institute. National Reference Centre for Bacterial Meningitis; Polonia.Fil: Sorhouet Pereira, Cecilia. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Departamento de Bacterilogía; Argentina.Fil: Stefanelli, Paola. Instituto Superiore di Sanitá. Department of Infectious Parasitic and Immune-Mediated Diseases; Italia.Fil: Tzanakaki, Georgina. National School of Public Health. National Meningitis Reference Laboratory; Grecia.Fil: Unemo, Magnus. Örebro University Hospital. National Reference Laboratory for Pathogenic Neisseria; Suiza.Fil: Vázquez, Julio A. Institute of Health Carlos III. Reference Laboratory for Neisserias; España.Fil: Vogel, Ulrich. University of Würzburg. Institute for Hygiene and Microbiology; Alemania.Fil: Wasko, Izabela. National Medicines Institute. National Reference Centre for Bacterial Meningitis; Polonia.Identification of clinical isolates of Neisseria meningitidis that are resistant to rifampin is important to avoid prophylaxis failure in contacts of patients, but it is hindered by the absence of a breakpoint for resistance, despite many efforts toward standardization. We examined a large number (n 392) of clinical meningococcal isolates, spanning 25 years (1984 to 2009), that were collected in 11 European countries, Argentina, and the Central African Republic. The collection comprises all clinical isolates with MICs of >0.25 mg/liter (n 161) received by the national reference laboratories for meningococci in the participating countries. Representative isolates displaying rifampin MICs of <0.25 mg/liter were also examined (n 231). Typing of isolates was performed, and a 660-bp DNA fragment of the rpoB gene was sequenced. Sequences differing by at least one nucleotide were defined as unique rpoB alleles. The geometric mean of the MICs was calculated for isolates displaying the same allele. The clinical isolates displaying rifampin MICs of >1 mg/liter possessed rpoB alleles with nonsynonymous mutations at four critical amino acid residues, D542, H552, S548, and S557, that were absent in the alleles found in all isolates with MICs of <1 mg/liter. Rifampin-susceptible isolates could be defined as those with MICs of <1 mg/liter. The rpoB allele sequence and isolate data have been incorporated into the PubMLST Neisseria database (http://pubmlst.org/neisseria/). The rifampin-resistant isolates belonged to diverse genetic lineages and were associated with lower levels of bacteremia and inflammatory cytokines in mice. This biological cost may explain the lack of clonal expansion of these isolates.