BACKGROUND AND OBJECTIVES: The serum hydroxyproline level (SHL) has been regarded as evidence of collagen breakdown or extra-cellular matrix reorganization. The role of SHL as a diagnostic parameter in acute coronary syndrome, and the SHL changes depending on the left ventricular remodeling after acute myocardial infarction, was evaluated. SUBJECTS AND METHODS: Venous blood samples were obtained from 122 patients with AMI at the time of admission, and on days 7, 21 and 90, and once from 15 patients with stable angina, 15 with unstable angina and 12 healthy subjects. The SHL was measured using the amino acid analysis system (HPLC). The regional wall motion index (RWMI), LVEF and LV dimensions were determined in the early (<1 week) and chronic (3 months) phases using echocardiography. Based on the final angiogram, patients were allocated to the reperfusion (TIMI II-III ; n=103) or non-reperfusion (TIMI 0-I ; n=19) groups. RESULTS: There were significant differences in the SHL among patients with AMI (6.49+/-3.77 microgram/L), unstable angina (2.92+/-1.81 microgram/L), stable angina (2.22+/-1.29 microgram/L) and the normal control groups (2.35+/-0.92 microgram/L). There was no significant difference in the SHL between the reperfusion and non-reperfusion groups on admission (p>0.05). However, there was a significant elevation in the SHL 14 day after AMI in the patients of the non-reperfusion group (4.36+/-1.46, 7.63+/-2.69 microgram/L, p=0.032). After 2 week, there was no significant difference in the SHL. There was a significant relationship between the late left ventricular ejection fraction and the SHL (r=-0.414, p=0.037), but no significant relationship between the other factors and the SHL (p>0.05). CONCLUSION: The serum hydroxyproline levels were significantly increased in patients with acute coronary syndrome, which reflect the early change in left ventricular remodeling in acute myocardial infarction.