The obstructive sleep apnea syndrome (OSAS) reduces attention span, memory and concentration capacities, all associated with cognition. The analysis of the auditory P300 parameters could help infer cognitive dysfunction. OBJECTIVE To compare the data from polysomnography and the auditory P300 in adults, primary snorers with OSAS patients. METHODS Prospective study with primary snorers (N=12) and in OSAS patients (N=54), submitted to polysomnography, defined by the apnea-hypopnea index (AHI). The polysomnography and P300 variables were compared by the t-Student test, the Exact Fisher's Test, logistic regression and analysis of correlation with a significance level of 5%. RESULTS AIH had an inverse correlation with the oximetry in both groups. The P300 prevalence was lower in the OSAS group (Fisher's Exact Test, p=0.027). Patient age did not influence the P300 prevalence (regression analysis; p=0.232). The P300 amplitude was lower in the OSAS group (t-Student test; p=0.003) and the P300 latency was similar in both groups (t-Student test; p=0.89). CONCLUSIONS The reduction in the P300 amplitude in patients with OSAS suggests cognitive dysfunction induced by a reduction in auditory memory.