To investigate determinants of daytime sleepiness in obstructive sleep apnea syndrome (OSAS), we studied 100 unselected OSAS patients by nocturnal polygraphic recording and the Multiple Sleep Latency Test (MSLT). Data obtained were submitted to three types of analysis. Respiratory disturbance index, oxygen saturation indices, body mass index, and total nocturnal sleep time did not significantly correlate with daytime sleepiness, as measured by the MSLT. Analysis of subgroups based on weight and degree of alertness also showed a nonsignificant correlation with daytime sleepiness. The best predictor of the excessive daytime sleepiness (EDS) frequently found in OSAS patients was the nocturnal polygraphic recording of the sleep disturbances and sleep structure anomalies that reflect the brain's overall dysfunction in OSAS. Understanding why an electroencephalogram arousal response occurs during sleep in association with abnormal breathing and how this response can become blunted may help us to better predict the development of EDS.