Alteration in myocardial collagen metabolism is an important factor in the progression of ventricular remodeling after myocardial infarction (MI). This study examined sequential changes in circulating levels of carboxyterminal propeptide of type I procollagen (PICP) as a collagen synthesis marker in order to assess the value of PICP for predicting the progression of left ventricular remodeling after MI. The study group comprised 20 patients with first MI undergoing reperfusion therapy. Peripheral blood samples were obtained on admission and serially up to 4 weeks after admission. Circulating levels of PICP and B-type natriuretic peptide (BNP), a tentative biochemical marker for the severity of MI, were measured by direct radioimmunoassay. Left ventricular end-diastolic volume index (EDVI) in acute and chronic phases were determined by left ventriculography, and changes (Δ) in EDVI were used as an index of left ventricular remodeling. Plasma PICP levels in the non-dilation group (< median ΔEDVI) showed no significant change. However, in the dilation group (> median ΔEDVI) PICP started to increase significantly 3 days after admission, peaking on day 14 (from 74 ± 6 to 104 ± 19 ng/ml, p < 0.05). ΔEDVI was significantly correlated with plasma PICP at 2 and 3 weeks, and with plasma BNP at 1, 2 and 3 weeks. Plasma PICP 2 weeks after MI was the only independent predictor of ΔEDVI (p < 0.001). These results suggest that an increase in plasma PICP levels 2 weeks after admission is a useful biochemical predictor of the progression of ventricular remodeling after MI.