Background: Women with multiple sclerosis (MS) experience an increased risk of relapse in the postpartum period. High-dose methylprednisolone is the first-line treatment for acute relapses. Objectives: To determine the transfer of methylprednisolone into human milk in breastfeeding MS patients. Methods: Methylprednisolone therapy was given for postpartum relapse to nine patients for three consecutive days, and seven patients received a daily infusion once a month. Breast milk samples were obtained before infusion and 1, 2, 4, 8, and 12 hours after completion of infusion. Results: Methylprednisolone concentrations in milk were below detection limits immediately before infusion. Cmax was measured at 1, 2, 4, 8, and 12 hours after infusion and levels of 2.100, 1.659, 0.680, 0.174, and 0.102 µg/mL were determined, respectively. The absolute infant dose was 98.98 µg/kg/day, and the relative infant dose (RID) was 0.71% of the weight-adjusted maternal dose. Conclusion: The level of methylprednisolone transfer into breast milk is very low. The RID for methylprednisolone was lower than the generally accepted value. As methylprednisolone therapy is of short duration, infant exposure would be very low should a mother choose to breastfeed 1 hour after infusion. Waiting 2–4 hours after infusion will limit infant exposure still further.