OBJECTIVE:To evaluate the influence of pregnancy and puerperium on the relapse rate of multiple sclerosis (MS). METHODS:We determined retrospectively the yearly mean relapse rate (MRR) during pregnancies occurring in the course of relapsing-remitting MS. We compared the MRR of pregnancy-time with that of non-pregnancy time by paired t-test. Relative risk (RR) of relapses during the pregnancy-time was also compared with that of non-pregnancy time by chi(2) analysis and 95% confidence intervals. RESULTS:From a population of 351 women affected by clinically definite MS, only 70 reported pregnancies during their relapsing-remitting phase of MS for a total of 98 pregnancies. Both MRR (P = 0.006) and RR (RR = 0.63, 95% CI = 0.40-0.94) decreased during the three trimesters of pregnancy. RR increased in the first 3 months of puerperium, although this was not statistically significant (RR = 1.36, 95% CI = 0.79-2.20). CONCLUSION: Our study confirms that in MS the relapse rate decreases throughout pregnancy and increases during puerperium. This suggests a complex interplay between hormonal and immune factors.