Despite sufficient continuous positive airway pressure (CPAP) therapy, some patients with the obstructive sleep apnea syndrome (OSAS) still suffer from excessive daytime sleepiness (EDS). In some of them, no cause of the persistence of EDS can be found. Brain damage due to nocturnal hypoxemia is a potential cause for this unclear persistent sleepiness (UPS). This study was done to evaluate this hypothesis. Patients with UPS were identified among the OSAS patients, who came for a CPAP therapy checkup to our sleep laboratory. UPS was recognized when no explanation for persistent EDS could be yielded by standard diagnostic procedures. Out of 167 patients under CPAP therapy 13 had UPS. To investigate the brain morphology, positron emission tomography (PET) scanning with the tracer fluorine-18 fluorodeoxyglucose (FDG), called FDG-PET, were performed in 7 of the UPS patients. Abnormal PET findings were concentrated in frontal area (found in 4 patients). The frontal abnormality seems to distinguish the OSAS patients with UPS from the whole OSAS population, examined in previous studies.