This study evaluated whether subclinical myocardial dysfunction occurs in first-degree relatives of patients with idiopathic dilated cardiomyopathy (IDCM), using strain echocardiographic imaging, before apparent left ventricular (LV) failure is observed. The study comprised 77 subjects aged 16–63 years who had first-degree relatives with a previous or new diagnosis of IDCM. LV myocardial deformation parameters of the first-degree relatives with normal LVEF (≥55%) values, as assessed using 2D echocardiography, were evaluated. The findings of the first-degree relatives were compared with an age- and sex-matched control group (n = 86). No difference in terms of age, gender, and body surface area was detected between first-degree relatives and controls. First-degree relatives of IDCM patients had significantly lower LVEF (62.04 ± 5.8% vs. 65.65 ± 6.3%, p