The objective of this study was to investigate whether the menstrual cycle influences multiple sclerosis (MS) symptoms. Seventy-two normally menstruating women (aged 20-50 years) with MS were interviewed. Of the 60 patients with a relapsing-remitting form of MS (RR-MS), 26 (43%) regularly experienced worsening of their MS symptoms in the period just before, or at the beginning of, the menstruation. Significantly more patients of the group reporting no influence of the premenstrual period on MS symptoms were using an oral contraceptive (p=0.041), suggesting a protective effect. There were 12 patients with the primary chronic progressive form of MS (PCP-MS). None of them experienced an influence of the menstrual cycle on their symptoms. Our results suggest that hormonal changes preceding menstruation may worsen symptoms in a subgroup of women with RR-MS.