Studies in the past have reported an increased risk of exacerbations in multiple sclerosis during the postpartum period; it is not known whether breast-feeding alters this risk. We interviewed 435 women regarding pregnancy and breast-feeding history, providing for analysis 191 pregnancies that had occurred during a nonprogressive phase of the disease. The exacerbation rates during the nine-month postpartum period (34%) were more than three times the exacerbation rate during the nine months of pregnancy (10%). The exacerbation risk was highest in the three-month period following childbirth and appeared to stabilize after the sixth postpartum month. The exacerbation rates in breast-feeding and non-breast-feeding pregnancies were 38% and 31%, respectively. The average time to exacerbation was similar in breast-feeding (3.0 months) and non-breast-feeding (3.1 months) pregnancies. Although differential exacerbation rates during pregnancy and the postpartum period may be related to hormonal effects on the immune system, the hormonal effects of breast-feeding do not appear to similarly affect the risk of exacerbation.