Subcutaneous interferon beta-1a is a recognized treatment of relapsing multiple sclerosis, and it may delay the onset of definite multiple sclerosis in patients with a first clinical demyelinating episode. Interferon beta-1a exerts beneficial effects on cognitive functioning via both short-term and long-term mechanisms. The beneficial effect is thought to be a result of immune modulation, with inhibition of leukocyte proliferation and antigen presentation and an increased amount of interleukins. The systemic side effects of interferon beta-1a are flu-like syndrome and development of neutralizing antibodies, the clinical significance of which is not known. There have been concerns about the rare development of an acute demyelinating disease after interferon beta-1a therapy as a result of upregulation of inflammatory mediators. In the clinical trials, there is evidence of development of mild anemia with a hemoglobin level below 10 g/dL only in 3% of the patients. There has been no reported case of development of aplastic anemia in patients being treated with interferon beta-1a. Described here is a case of development of aplastic anemia with interferon beta-1a in a patient with multiple sclerosis. Our patient underwent a complete hematologic evaluation to rule out other causes of aplastic anemia. Association with interferon beta-1a was highly suspected.