The role of fish consumption and omega 3 supplementation in multiple sclerosis (MS) is controversial, although there is some evidence to support a beneficial effect. We surveyed a large cohort of people with MS recruited via Web 2.0 platforms, requesting information on type of MS, relapse rates, disability, health-related quality of life, frequency of fish consumption and omega 3 supplementation, including type and dose, using validated tools where possible. We aimed to determine whether there was an association between fish consumption and omega 3 supplementation and quality of life, disability and disease activity for people with MS. Univariate and multivariate analyses were undertaken. Of 2469 respondents, 1493 (60.5%) had relapsing-remitting MS. Those consuming fish more frequently and those taking omega 3 supplements had significantly better quality of life, in all domains, and less disability. For fish consumption, there was a clear dose-response relationship for these associations. There were also trends towards lower relapse rates and reduced disease activity; flaxseed oil supplementation was associated with over 60% lower relapse rate over the previous 12 months. Further dietary studies and randomised controlled trials of omega 3 supplementation for people with MS are required, preferably using flaxseed oil.