Objective: To evaluate the usefulness of heart rate variability analysis (HRV) analysis from the long duration electrocardiogram (ECG) recordings as a screening test for the diagnosis of moderate-to-severe obstructive sleep apnea syndrome (OSAS). Methods: Recordings from 87 patients who were admitted to sleep laboratory for polysomnographic study (PSG) were evaluated. Finally 30 cases with apnea-hypopnea index (AHI)≥15/hour included in patient’s group (male/female: 22/8; mean age: 49±10 years) and 21 cases with AHI<5 were included in control group (male/female: 10/11; mean age: 48±11 years). From the ECG recordings taken as a part of PSG, time -domain and fre- quency -domain HRV parameters were evaluated and their accuracy in the diagnosis of OSAS was investigated using ROC analysis. Results: Statistically significant differences were found in HRV variables in time- domain parameters such as SDNN, SDNN index; frequency- domain parameters VLF, LF, nuLF, nuHF, LF/HF and geometric parameter HRV triangular index values in between groups. Cut- off value of 16 for the HRV triangular index was found to be 50% sensitive and 85.7% specific with a positive likelihood ratio (LR) of 3.50% and negative LR of 0.58% When the total power was higher than 9.611, then the analysis sensitivity was 53.3%, specificity was 95.6%, positive LR was 11.2% and negative LR was 0.49% When the SDNN was higher than 83 then its sensitivity was 80%, specificity was 76.2%, positive LR was 3.36% and negative LR was 0.26% For the cut off value of 62 calculated for SDNN index, sensitivity was 73.3%, specificity was 85.7%, positive LR was 5.13% and nega- tive LR was 0.31%, for the cut off value of 9.12 was calculated for VLF, sensitivity was 90.4% specificity was 50% positive LR was 1.81% and negative LR was 0.19%. Conclusion: Heart rate variability analysis done over routine single channel ECG data gathered through routine Holter applications may be help- ful in distinguishing moderate-to-severe OSAS patients from mild OSAS patients and non-OSAS control subjects.