Pathological left ventricular hypertrophy (LVH) with myocardial fibrosis is an independent risk factor for cardiovascular mortality. Previous studies indicated that patients with coarctation of the aorta (CoA) have increased left ventricular mass (LVM) including LVH, even after successful CoA repair. It is unclear whether the increased LVM is pathological one with cardiac fibrosis. Group A consisted of 17 patients with successfully repaired CoA. Group B consisted of 17 postoperative subjects who matched the age and postoperative periods of group A. Group C comprised 28 subjects for the geometric standard of the left ventricle. The LVM index (LVMI) and the relative wall thickness (RWT) of group A and B were compared with the values of 17 age-matched subjects from group C. The serum concentration of procollagen type III amino-terminal peptide (P-III-P), a biomarker for myocardial fibrosis, in group A was compared with the concentration in group B. The correlations between the serum P-III-P concentration and LVMI and RWT were studied in group A and non-A group. In group A, RWT and LVMI were significantly higher than those in group C (0.37 ± 0.05 vs. 0.31 ± 0.02, p < 0.01; 44.8 ± 11.2 vs. 36.5 ± 7.6, p = 0.04, respectively), and the serum P-III-P concentration was significantly higher than that in group B (1.59 ± 0.74 vs. 1.07 ± 0.33, p = 0.04). Serum P-III-P concentrations were well correlated with RWT and LVMI (r = 0.89, p < 0.01; r = 0.63, p < 0.01, respectively) in group A. LVH in patients with successfully repaired CoA may have an abnormal pathogenesis associated with myocardial fibrosis.