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Background: amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of upper and lower motor neurons, leading to progressive weakness of voluntary muscles, with death following from neuromuscular respiratory failure, typically within 3 to 5 years. There is a strong genetic contribution to ALS risk. In 10% or more, a family history of ALS or frontotemporal dementia is obtained, and the Mendelian genes responsible for ALS in such families have now been identified in about 50% of cases. Only about 14% of apparently sporadic ALS is explained by known genetic variation, suggesting that other forms of genetic variation are important. Telomeres maintain DNA integrity during cellular replication, differ between sexes, and shorten naturally with age. Sex and age are risk factors for ALS and we therefore investigated telomere length in ALS. Methods: samples were from Project MinE, an international ALS whole genome sequencing consortium that includes phenotype data. For validation we used donated brain samples from motor cortex from people with ALS and controls. Ancestry and relatedness were evaluated by principal components analysis and relationship matrices of DNA microarray data. Whole genome sequence data were from Illumina HiSeq platforms and aligned using the Isaac pipeline. TelSeq was used to quantify telomere length using whole genome sequence data. We tested the association of telomere length with ALS and ALS survival using Cox regression. Results: there were 6,580 whole genome sequences, reducing to 6,195 samples (4,315 from people with ALS and 1,880 controls) after quality control, and 159 brain samples (106 ALS, 53 controls). Accounting for age and sex, there was a 20% (95% CI 14%, 25%) increase of telomere length in people with ALS compared to controls (p = 1.1 × 10?12), validated in the brain samples (p = 0.03). Those with shorter telomeres had a 10% increase in median survival (p = 5.0×10?7). Although there was no difference, European Community's Health Seventh Framework Program (FP7/2007-2013); European Union (EU); Horizon 2020; H2020-PHC2014-two-stage; European Research Council (ERC); Research and Innovation Programme; AAK was funded by ALS Association Milton Safenowitz Research Fellowship (grant number 22-PDF-609. doi: 10.52546/pc.gr.150909), The Motor Neurone Disease Association (MNDA) Fellowship (Al Khleifat/Oct21/975799), The Darby Rimmer Foundation, and The NIHRMaudsley Biomedical Research Centre. This project was also funded by the MND Association and the Wellcome Trust. This is an EU Joint Programme-Neurodegenerative Disease Research JPND) project. The project is supported through the following funding organizations under the aegis of JPND-www.jpnd.eu [United Kingdom, Medical Research Council (MR/L501529/1 and MR/R024804/1) and Economic and Social Research Council (ES/L008238/1)]. AA-C was a NIHR Senior Investigator. CS and AA-C received salary support from the National Institute for Health Research (NIHR) Dementia Biomedical Research Unit at South London and Maudsley NHS Foundation Trust and King's College London. The collaboration project was co-funded by the PPP Allowance made available by Health ~Holland, Top Sector Life Sciences and Health, to stimulate public-private partnerships. Project MinE Belgium was supported by a grant from IWT, the Belgian ALS Liga and a grant from Opening the Future Fund (KU Leuven). PVD holds a senior clinical investigatorship of FWO-Vlaanderen and was supported by E. von Behring Chair for Neuromuscular and Neurodegenerative Disorders, the ALS Liga Belgie and the KU Leuven funds ""Een Hart voor ALS,"" ""Laeversfonds voor ALS Onderzoek,"" and the ""Valery Perrier Race against ALS Fund"". RM was supported by Science Foundation Ireland (17/CDA/4737). MinE USA was funded by the US ALS Association.