The purpose of this study was to determinate the prevalence of undiagnosed diabetes mellitus (UDM) using glycated hemoglobin (GHb) among functionally dependent and severe cognitive depressed elderly long-term care (LTC) patients received enteral nutrition. The patients were divided into three categories: control, patients with (diabetes mellitus, DM) and without previously diagnoses (UDM) of DM. Demographic, functional and cognitive status, laboratory data and rate of death were recorded. Patients with HbA-1c level of 7% and more were diagnosed as diabetic. On the baseline, 26% of the patients presented with known DM. Patients from control and previously UDM groups presented with similar fasting plasma glucose (FPG) and post-prandial glucose (PPG) levels, but were different by HbA-1c levels (p<0.001). In 79% of patients suffering from UDM, HbA-1c levels was more than 7%, and in 24% of them more than 8%. Prevalence of UDM was 21%. Patients from UDM were presented as borderline group between Control group (C-G) and DM. Those differences presented by all general characteristics of the patients: age, functional and cognitive status, BMI, comorbidity, laboratory data and rate of death. This study suggests that 47% of LTC patients who received enteral nutrition suffered from DM, and 44% of them were undiagnosed. The authors recommend that the GHb is a simple and an effective laboratory analysis in detection of DM in this group of patients.