Adaptive enlargement of the mitral leaflet has been implied to participate in the pathogenesis of functional mitral regurgitation (FMR). The aim of the present study was to observe the elongation pattern of anterior mitral leaflets (AML) and posterior mitral leaflets (PML) in idiopathic dilated cardiomyopathy (DC) and to explore its relation with FMR. Forty normal controls (control group) and 97 patients with idiopathic DC (group DC 0-1+: 36 patients with no or only mild FMR; group DC >1+: 61 patients with more-than-mild FMR) were consecutively recruited. The lengths of AML and PML were measured at the parasternal long-axis view (AML-lax, PML-lax) and apical 4-chamber view (AML-4, PML-4) using 2-dimensional echocardiography, as well as tenting height (TH) and mitral annular dimension (MAD). Both AML (AML-lax: 2.4 ± 0.3 vs 3.0 ± 0.3 vs 3.1 ± 0.3 cm; AML-4: 1.9 ± 0.2 vs 2.5 ± 0.3 vs 2.6 ± 0.4 cm) and PML (PML-lax: 1.3 ± 0.3 vs 2.1 ± 0.5 vs 2.5 ± 0.4 cm; PML-4: 1.1 ± 0.2 vs 1.6 ± 0.3 vs 1.8 ± 0.4 cm) were elongated in the DC groups compared to controls (all p <0.001). There was a further elongation of PML in group DC >1+ than in group DC 0-1+ (p <0.05), but the AML length was not different (p >0.05). The ratio of (AML-lax + PML-lax)/(TH-lax + MAD-lax) (1.03 ± 0.10 vs 1.08 ± 0.09, p <0.05) or AML-lax/(TH-lax + MAD-lax) (0.57 ± 0.06 vs 0.64 ± 0.08, p <0.001) in group DC >1+ was significantly smaller compared to group DC 0-1+, whereas the ratio of PML-lax/(TH-lax + MAD-lax) was similar between the 2 groups (0.46 ± 0.06 vs 0.44 ± 0.07, p = 0.138). In conclusion, both the AML and PML were elongated in idiopathic DC, but the extent and pattern were not identical between the 2 leaflets. Inadequate AML elongation proportional to mitral apparatus remodeling more likely contributes to the pathogenesis of FMR.