Erythroblast appearance associated with natalizumab

•NTZ treatment led to erythroblast appearance in peripheral blood.•Continuous administration was possible after the appearance of erythroblasts.•Natalizumab may inhibit erythroblast maturation by preventing erythroblastic island formation. Despite having a high rate of occurrence, erythroblast appearance in peripheral blood may not be a recognized adverse effect of natalizumab (NTZ) treatment. Additionally, the time course and cause of erythroblast appearance remain unclear. We report two cases of multiple sclerosis wherein NTZ treatment led to erythroblast appearance in peripheral blood. Erythroblasts appeared after NTZ administration; however, their counts did not increase and the administration of medication was continued. NTZ can inhibit erythroblastic island formation associated with maturing of erythroblast via VLA-4. Clinicians do not need to be afraid; however, careful observation is recommended because some patients may develop anemia.