Case Report: Clinicians diagnosed hairy cell leukemia in a man aged 57 years in 2007. He was never treated because the disease was indolent with only moderate thrombocytopenia (platelet count, 85 × 109 cells/L). We saw him in November 2008 for elevated liver enzyme levels (alanine aminotransferase, 135 IU/L; aspartate aminotransferase, 80 IU/L; and γ-glutamyltransferase, 311 IU/L). We ruled out all other causes of acute and chronic liver disease and detected HEV RNA in the patient's serum and stools. Test results for anti-HEV IgG and IgM were positive (Adaltis, Ingen, France), and the HEV genotype was 3c. We performed a liver biopsy in December 2008 and found mild lobular hepatitis without fibrosis. The leukocyte count was 4.3 × 109 cells/L, and the hairy cell leukocyte count was 0.45 × 109 cells/L. The CD4+ cell count was normal (0.582 × 109 cells/L). We detected no monocytes in the peripheral blood. After 1-year follow-up without therapy, we started the patient on a 3-month course of pegylated interferon-α2b, 1 µg/kg of body weight per week (Figure). The serum HEV RNA concentration decreased from 5.6 log10 copies/mL at baseline to 2.4 log10 copies/mL by week 2, and the patient achieved a complete virologic response by week 4 (Figure). At week 7, liver enzyme levels remained within the normal range, and we could not detect HEV RNA in stools (lower limit of detection, 200 copies/g). We stopped antiviral treatment in November 2009 and could not detect serum HEV RNA after 5 months (lower limit of detection, 200 copies/mL) (Figure).