We aimed to evaluate the importance of neck/thyromental distance in the diagnosis of moderate to severe obstructive sleep apnea (OSA) in sleep clinics. 185 patients (122 males, 63 females) referred to our sleep clinic with OSA symptoms were enrolled to the study. The patients had level-1 polysomnography (PSG). The neck circumference (N), thyromental distance (T), and STOP test were recorded in all patients. Using an obstructive AHI > 15 event/h on PSG as the cut-off, the best N/T ratio to find patients with OSA was calculated with the receiver operator curve analyses. The best cut-off for N/T was chosen as 4.6. We used Modified STOP test: STO-NT test in which P (for hypertension item) was replaced with N/T ratio. N/T ratio >4.6 was scored as "positive". Two positives out of four questions in STO-NT were scored as high risk for OSA. The OSA prevalence was 60 % for AHI > 15. The mean ratio of N/T was significantly different between groups with AHI > 15 and AHI ≤ 15. N and N/T ratio were moderately correlated with AHI. Sensitivity, specificity, negative predictive value, positive predictive value, and negative likelihood ratio of STOP test for AHI > 15 were 88.5, 28.4, 61.8, 65.4 % and 0.40, whereas 97.3, 23, 85, 65.9 % and 0.12 for STO-NT test, respectively. STO-NT test seems better than STOP test in determining patients who do not likely to have moderate to severe OSA in sleep clinics so can be preferred to decide on therapies other than CPAP in a short time.