Multiple sclerosis (OMIM 126200) is a common disease of the central nervous system in which the interplay between inflammatory and neurodegenerative processes typically results in intermittent neurological disturbance followed by progressive accumulation of disability. 1 Epidemiological studies have shown that genetic factors are primarily responsible for the substantially increased frequency of the disease seen in the relatives of affected individuals; 2,3 and systematic attempts to identify linkage in multiplex families have confirmed that variation within the Major Histocompatibility Complex (MHC) exerts the greatest individual effect on risk. 4 Modestly powered Genome-Wide Association Studies (GWAS) 5-10 have enabled more than 20 additional risk loci to be identified and have shown that multiple variants exerting modest individual effects play a key role in disease susceptibility. 11 Most of the genetic architecture underlying susceptibility to the disease remains to be defined and is anticipated to require the analysis of sample sizes that are beyond the numbers currently available to individual research groups. In a collaborative GWAS involving 9772 cases of European descent collected by 23 research groups working in 15 different countries, we have replicated almost all of the previously suggested associations and identified at least a further 29 novel susceptibility loci. Within the MHC we have refined the identity of the DRB1 risk alleles and confirmed that variation in the HLA-A gene underlies the independent protective effect attributable to the Class I region. Immunologically relevant genes are significantly over-represented amongst those mapping close to the identified loci and particularly implicate T helper cell differentiation in the pathogenesis of multiple sclerosis. ; PUBLISHED ; The principal funding for this study was provided by the Wellcome Trust (085475/B/08/Z, 085475/Z/08/Z, 075491/ Z/04/Z and 068545/Z/02). The work was also supported by National Institutes of Health (AI076544, NS032830, NS049477, NS19142, NS049510, NS26799, NS43559, NS067305, CA104021, RR020092, RR024992 and K23N/ S048869), US National Multiple Sclerosis Society (RG 4201-A-1), Nancy Davis Foundation, Cambridge NIHR Biomedical Research Centre, UK Medical Research Council (G0700061, G0000934), Multiple Sclerosis Society of Great Britain and Northern Ireland (898/08), Wolfson Royal Society Merit Award, Peter Doherty fellowship, Lagrange Fellowship, Harry Weaver Neuroscience Scholarships, Australian National Health and Medical Research Council (NHMRC), Australian Research Council Linkage Program Grant, JHH Charitable Trust Fund, Multiple Sclerosis Research Australia, Health Research Council New Zealand, National MS Society of New Zealand, Wetenschappelijk Onderzoek Multiple Sclerose, Bayer Chair on Fundamental Genetic Research regarding the Neuroimmunological aspects of Multiple Sclerosis, Biogen Idec Chair Translational Research in Multiple Sclerosis, FWO-Vlaanderen, Belgian Neurological Society, Danish Multiple Sclerosis Society, Neuropromise EU grant (LSHM-CT-2005-018637), Center of Excellence for Disease Genetics of the Academy of Finland, Sigrid Juselius Foundation, Helsinki University Central Hospital Research Foundation, Bundesministerium für Bildung und Technologie (KKNMS consortium Control MS), Deutsche Forschungsgemeinschaft, Institut National de la Santé et de la Recherche Médicale (INSERM), Association pour la Recherche sur la Sclérose En Plaques (ARSEP), Association Française contre les Myopathies (AFM), Italian Foundation for Multiple Sclerosis (2002/R/40, 2005/R/ 10, 2008/R/11 and 2008/R/15), Italian Ministry of Health (grant Giovani Ricercatori 2007 - D.lgs 502/92), Regione Piemonte (grants 2003, 2004, 2008, 2009), CRT Foundation, Turin, Moorfields / UCL Institute of Ophthalmology NIHR Biomedical Research Centre, Norwegian MS Register and Biobank, Research Council of Norway, South- Eastern and Western Norway regional Health Authories, Ullevål University Hospital Scientific Advisory Council, Haukeland University Hospital, Amici Centro Sclerosi Multipla del San Raffaele (ACESM), Association of British Neurologists, Spanish Ministry of Health(FISPI060117), Bibbi and Niels Jensens Foundation, Montel Williams foundation, Hjärnfonden and Swedish medical research council (8691), Stockholm County Council (562183), Swedish Council for Working life and Social Research, Gemeinnützige Hertie Stiftung, Northern California Kaiser Permanente members and Polpharma Foundation, and Washington University Institute of Clinical and Translational Sciences - Brain, Behavioral and Performance Unit