The effect of pregnancy on the relapse rate of multiple sclerosis (MS) has been investigated. Obstetric and relapse histories of 52 women from New South Wales with clinically definite MS were obtained by telephone interview. These women had 101 pregnancies of which 85 were at risk of an MS relapse. No increased risk of relapse during the pregnancy period was demonstrated. The relapse that did occur during the pregnancy period were clustered in the last trimester of pregnancy and first 3 months post partum.