Gene expression profiles of paired hepatocellular carcinoma (HCC) and adjacent noncancerous liver tissue samples revealed preferential expression of midkine in HCC. This finding suggested the clinical usefulness of midkine measurement in serum for monitoring HCC treatment response, recurrence, and progression. A prospective study in 285 patients, 144 in complete remission and 141 at risk for developing de novo HCC, was conducted. The changes in serum midkine level were in parallel with disease activity in about 81% of patients with HCC. The study also revealed that rapidly rising serum midkine levels occurred in patients in the terminal stage of HCC. The rising rate of serum midkine levels was inversely correlated with remaining survival days. However, serum midkine measurement did not detect emergence of new HCC in most patients in complete remission and in high-risk people without a history of HCC. Serum midkine levels can be useful to monitor HCC progression, and a sharp rise signals the approach of end of life in patients with HCC.