Interferon β-1a is a widely used immunomodulation treatment for multiple sclerosis. Liver function impairment is a common side effect and usually develops in the first 6 months after interferon use. Here, we describe 2 multiple sclerosis patients who developed delayed liver function impairment 5 years after receiving interferon β-1a treatment. Their liver function recovered after discontinuing interferon use, and further detailed hepatological evaluations excluded other etiologies of liver function impairment. Our case reports illustrate that liver function impairment induced by interferon β-1a can be delayed for 5 years after starting treatment and, probably, this is an idiosyncratic reaction. Regular liver function monitoring in multiple sclerosis patients who receive interferon β is necessary even after the first 6 months of treatment, especially in those patients with concomitant use of other liver-toxic medications.