The aim of the research was to discover the level of change observed in cognitive functions and personality variables in patients ill with obstructive sleep apnea prior to and after CPAP treatment. Data was collected from 20 male patients ill with a severe form of obstructive sleep apnea whose physiological symptoms were correlated with the results of cognitive functions and personality variables. It was concluded that obstructive sleep apnea patients have lowered; attention, concentration, speed of learning verbal and visual stimuli and speed of learning visual-motor tasks. An increased level of depression was noted together with increased anxiety and psychological tension. In half of the group irregular EEG recordings and a pathological result on the Benton Test suggests organic damage to the CNS. These changes were negatively correlated with amounts of REM and 3 + 4 NREM sleep, but were positively correlated with the level of sleep hypoxemia. The results suggest that after 3 months of CPAP treatment affect improved significantly. The observed tendency for cognitive functions to improved was not statistically significant.