Polysomnography is currently considered as the gold standard for the diagnosis of obstructive sleep apnea syndrome (OSAS). But high expense and the backlog of the sleep centers have resulted in a search for an alternative method of diagnosis. The aim of this study is to assess the efficacy and reliability of Watch PAT as an alternate option in OSAS diagnosis. The patients have worn a Watch PAT(A (R)) 200 device in the sleep laboratory during a standard polysomnography. The correlation in REM and Non-REM AHI scores, sleep periods and the mean O-2 saturation percentage between Watch PAT and PSG sleep studies were assessed. There was a statistically significant very strong correlation between PSG and Watch PAT AHI scores (Spearman's rho = 0.802 p < 0.001). The mean recording time with PSG and Watch PAT was 463.06 +/- A 37.08 and 469.33 +/- A 72.81 min, respectively, and there was no statistically significant difference (p = 0.068). However, there was a statistically significant difference between two methods regarding the average sleep time and REM sleep period. No statistically significant difference was revealed in the mean O-2 saturation percentage (p < 0.001). Watch PAT is an efficient device and is considered to be an adjunctive diagnostic method for PSG in diagnosis of OSAS.