Background: Sleep may be disrupted in Multiple Sclerosis (MS), but the prevalence of chronic insomnia disorder (CID) using standard diagnostic criteria is unknown. Objectives: To determine the prevalence of CID in an MS population, the frequency of CID subtypes, associated factors and impact on quality of life (QoL). Methods: Multicentre, hospital-based cross-sectional study. An adapted version of the Brief Insomnia Questionnaire was applied to a consecutively recruited MS population. The influence of demographic, MS-related features, fatigue, medical and psychiatric comorbidities, nocturnal symptoms, other sleep disorders, dysfunctional beliefs about sleep in CID was evaluated. The relation between CID and QoL was analysed. Results: Of 206 MS patients, 22.3\% fulfilled criteria for CID, with initial insomnia in 11.7\%, maintenance insomnia in 11.2\% and terminal insomnia in 10.2\% of patients. CID was more frequent in female patients, those with nocturnal symptoms, medical comorbidities, higher levels of anxiety, depression and fatigue. Multivariable analysis identified female sex, medical comorbidities, anxiety and fatigue as independent factors for CID. CID patients had a significantly lower self-reported QoL. Conclusions: CID is prevalent in MS patients and associated with psychiatric and medical comorbidities, as well as fatigue. It has a negative impact on QoL. (C) 2015 Elsevier B.V. All rights reserved.