Aim To compare cardiac magnetic resonance imaging (MRI) features between isolated left ventricular non-compaction (IVNC) and dilated cardiomyopathy (DCM) in adults. Materials and methods A consecutive series of 50 patients with IVNC from a single institution were reviewed. During the same period, 50 patients with DCM who had prominent trabeculations, who were matched for age, gender, and body surface area, were prospectively included. Left ventricular (LV) morphology and function were assessed using cardiac MRI. Results Compared with patients with DCM, patients with IVNC had a significantly lower LV sphericity index and end-diastolic volume index (LVEDVI) and a greater LV ejection fraction (LVEF), number of trabeculated segments, and ratio of non-compacted to compacted myocardium (NC/C ratio). There were no significant differences in stroke volume index, cardiac output, and cardiac index between the two patient groups. In patients with IVNC, the number of trabeculated segments and the NC/C ratio correlated positively with LVEDVI ( r = 0.626 and r = 0.559, respectively) and negatively with LVEF ( r = −0.647 and r = −0.521, respectively, p < 0.001 for all). In patients with DCM, the number of non-compacted segments and the NC/C ratio had no correlation with either the LVEDVI ( r = −0.082 and r = −0.135, respectively) or the LVEF ( r = 0.097 and r = 0.205, respectively). Conclusion There are demonstrable morphological and functional differences between IVNC and DCM at LV assessment using cardiac MRI. The occurrence of trabeculated myocardium might be due to a different pathophysiological mechanism.