Background: The aim of the study is to elucidate the association between changes in sleep EEG and alteration of cognitive functions in patients with ischemic stroke. Methods: We studied 11 consecutive patients with first-ever hemispheric ischemic stroke admitted within 3 days after symptoms onset. Stroke volume was assessed by MR imaging. Sleep EEG and nocturnal breathing were examined with a validated portable system. Cognitive function was evaluated using a standardized battery of tests. A second examination of sleep EEG and cognitive function was performed in 9 patients at least 3 months after stroke. The patients were compared with 5 gender- and age-matched healthy controls. Results: Mean age was 43±12 years (range 18–60), 4 were males. In 5 patients stroke were right-sided, in 6 left-sided. Mean volume of the stroke lesion was 82±66 ml (range 5–200). In the acute stroke phase a significant correlation between (1) sleep efficiency and attention (p=0.039) and (2) amount of REM sleep and verbal fluency (p=0.016) was found. A significant improvement of sleep efficiency (p=0.005), verbal fluency (p=0.009) and non verbal fluency (p=0.02) was found in the control examination. In the chronic phase we observed a correlation between amount of REM sleep and visual learning ability (p=0.017). Conclusions: The results suggest the presence of a link between changes in sleep EEG pattern and alteration of cognitive functions during the acute and the recovery phase after hemispheric stroke.