Background A prospective 2-night polysomnographic (PSG) study in Chinese snorers was designed to assess the role of the first night effect (FNE) in PSG parameters and the diagnosis of obstructive sleep apnea hypopnea syndrome (OSAHS). Methods Seventy-two snorers from two teaching hospitals underwent overnight PSG on two consecutive nights. The night-to-night variability of PSG parameters were recorded and analyzed. Results Sixty-six patients were analyzed. Among all the PSG parameters, only the total time of stage 2 presented a significant difference between two nights: 219.50 (83.50-353.50) vs. 215.25 (59.50-342.50) (P=0.000). Subgroup assessment showed a slight night-to-night difference in about 1-2 parameters in the group with the apnea-hypopnea index (AHI) >= 20 events per hour as well as the group with AHI <20 events per hour, but there was no night-to-night difference in the AHI in each group. And slighter FNE was found among patients >= 40 years old. There was no significant difference in diagnosis of OSAHS. In the decision of severity, a slight difference was found between the two nights with a Kappa value=0.531. Conclusions Only mild FNE can be found on two consecutive nights of PSG in adult Chinese snorers, but it has no effect on the diagnosis of OSAHS. A single polysomnographic night may be adequate for the diagnosis of OSAHS. Chin Med J 2011;124(24):4127-4131