Data from the Oxford.FPA prospective study show that oral contraceptive use and pregnancy have no discernible effect on the risk of developing multiple sclerosis (MS). Women of parity 0-2 developed MS twice as often as women of parity 3 or more but the difference did not reach statistical significance. Smoking may be a risk factor for developing MS. A nested case-control analysis did not identify any associations between MS onset and preceding illnesses.