We measured serum thrombopoietin (TPO) in chronic hepatitis C treated with interferon (IFN). The platelet count before the therapy was 161.9 x 10(9) +/- 64.1 x 10(9)/l, which decreased to 116.3 x 10(9) +/- 48.4 x 10(9)/l 1 week after IFN therapy (P < 0.01). On the other hand, serum TPO increased from 1.96 +/- 0.60 fmol/ml to 2.68 +/- 0.69 fmol/ml (P < 0.02). Contrary to a recent report that serum TPO was not altered in liver cirrhosis, these data indicate that serum TPO was increased in chronic hepatitis C in response to thrombocytopenia by IFN therapy.