The effect of pregnancy on the relapse rate of multiple sclerosis has been investigated. Obstetric and relapse histories of 52 women with clinically definite multiple sclerosis were obtained; there were 101 pregnancies of which 85 were at risk of a multiple sclerosis relapse. By comparing the number of relapses in the pregnancy period (9 months of pregnancy and 6 months immediately post partum) with the number in non-pregnancy periods for the same women, no increased risk of relapse during the pregnancy period was demonstrated. However, of the relapses that did occur during the pregnancy period, there was a significant reduction in the number of relapses in the first and second trimesters.