['A previous study identified midkine (MK) expression in primary gastrointestinal stromal tumor (GIST) as a prognostic marker. The aim of the current study was to compare serum midkine (S-MK) concentrations of GIST patients with those of healthy controls and to determine if MK can serve as a prognostic serum marker for these patients.', 'S-MK concentrations were measured by enzyme-linked immunosorbent assay in GIST patients (n\xa0=\xa096) and healthy controls (n\xa0=\xa0148). S-MK levels were then correlated with clinicopathological data and the administration of imatinib therapy. In addition, MK expression was evaluated in 39 surgically resected GIST and in 17 leiomyoma specimens on a tissue microarray.', 'S-MK concentrations in GIST patients were significantly higher than in healthy controls: median (25th and 75th percentiles) S-MK concentration was 235 (139 and 376) pg/ml in the GIST patients and 99 (33 and 198) pg/ml in the controls (P\xa0<\xa00.001; Mann-Whitney U test). Significantly higher median S-MK concentrations were found in GIST with recurrence compared with those without (295 vs 230; P\xa0=\xa00.009). GIST patients with S-MK levels higher than 400\xa0pg/ml showed a significantly worse recurrence-free survival (P\xa0=\xa00.026; log-rank test). Patients receiving imatinib therapy had decreased median S-MK concentrations compared with those who were not treated with imatinib (331 vs 201; P\xa0<\xa00.001).'] Does serum midkine correlate with tumor progression and imatinib response in gastrointestinal stromal tumors?