The influence of pregnancy on multiple sclerosis was studied in 338 women by determining in each trimester of pregnancy and post partum the number of relapses and the corresponding relapse rate. Eighty-five relapses occurred in association with 199 pregnancies, most (65) in the postpartum period, and a low number of relapses (2) were recorded in the last trimester of pregnancy. Comparing the average exacerbation rate of the study group with that of patients with multiple sclerosis in Israel (0.28 relapses per person per year), we found a statistically significant decrease in the third trimester (0.04) and a high increase in the first three months post partum (0.82). This pattern of remissions at the end of pregnancy and exacerbations post partum is similar to that observed in other putative autoimmune diseases.