Vitamin D (VD) deficiency is a common disease that occurs in all stages of life. A growing number of studies call attention to the relationship between VD deficiency and cardiovascular disease. The aim of this study was to investigate the effect of VD on subclinical left ventricular (LV) function in diabetic and non-diabetic patients with no significant coronary artery disease. We recruited 140 patients (80 diabetics and 60 non-diabetics) with symptoms of stable ischemic heart disease who underwent coronary angiography and who had no significant coronary artery disease in our clinic. The 25(OH)D3 levels were measured and patients who had 25-(OH)D3 levels below 20 ng/dl were defined as the VD deficient group. In addition to conventional echocardiographic parameters, tissue Doppler echocardiography was used for LV diastolic functions and 2D speckle tracking strain echocardiography (2D STE) for evaluating the longitudinal deformation indices of the LV myocardium. In all groups, LV global longitudinal strain (GLS) was significantly impaired in patients with VD deficiency (p < 0.001) compared to patients without VD deficiency. LV global longitudinal strain rate (GLSR) was significantly impaired in patients with VD deficiency (p = 0.003). The GLS was negatively associated with 25-(OH)D3 in the VD deficiency group (r = -0.52623, p < 0.001). Conversely, GLS was positively associated with 25-(OH)D3 levels in the normal VD group (r = 0.28, p = 0.048). VD deficiency is associated with impaired myocardial GLS. The present study demonstrated that VD deficiency may be the cause of subclinical myocardial dysfunction in patients with or without diabetes mellitus and no history of significant coronary artery disease. Copyright: © 2020 Termedia & Banach.