Epidemiological studies have demonstrated that environmental influences contribute to determining the risk of multiple sclerosis (MS). The nature of this influence has not been established, although infectious agents have received the most attention with relative neglect of alternative hypotheses. This paper critically reviews the evidence implicating dietary fat in altering susceptibility to MS. It is concluded that a dietary theory accords with current knowledge regarding MS as well as an infective theory and thus should not be dismissed in research examining the aetiology of MS.