BACKGROUND The aim was to examine and compare the characteristics of intra- and inter-hemispheric EEG coherence in patients with mild cognitive impairment (MCI) and healthy controls at rest and during a working memory (WM) task having three levels. METHODS The subjects included 35 patients with MCI according to the DSM-IV criteria (mean age: 62.3+/-6.5 years) and 34 healthy controls (mean age: 57.4+/-4.0) selected from the community. All the subjects performed a simple calculation and recall task of WM with three levels of (WM1-3) while their EEG signal was recording. Inter- and intra-hemispheric EEG coherence was analyzed during the WM loads. RESULTS Intra- and inter-hemispheric coherence during the WM task showed a "drop-to-rise" tendency compared with that at-rest condition. There was significantly higher coherence in MCI patients than in controls during the WM task. Intra-hemispheric ortical connectivity diminished while inter-hemispheric connectivity dominated in the MCI patients during WM with increasing task difficulty. CONCLUSIONS MCI patients and normal controls may have differences in cortical connection and MCI patients have a cortical connection disturbance during WM tasks. With increasing task difficulty the MCI patients mobilized more inter-hemispheric than intra-hemispheric cognitive resources.