Neuroimaging studies of cholinesterase inhibitor (ChEI) treatment in Alzheimer's disease (AD) have shown the different short and long term actions of ChEIs. fMRI studies of the ChEI donepezil have focused on its short to medium term action without exploring the effects of established treatment. In this exploratory study the effect of 20 weeks donepezil treatment on regional brain activity was measured with fMRI in patients with mild AD. Twelve patients with probable AD and nine age-matched controls were assessed with a Pyramids and Palm Trees semantic association fMRI paradigm and an n-back working memory fMRI paradigm. In the patient group only, the assessment was repeated after 20 weeks of treatment. After treatment, differences from normal healthy elderly became more pronounced. There was also a spread of deactivation which at retest was detectable in task relevant areas. Behaviourally, however, there were no significant differences between group baseline and retest scores, with a range of performance probably reflecting variation in drug efficacy across patients. Parametric analyses established that increased behavioural scores at retest correlated significantly with higher activation levels in non task relevant areas. Behavioural stability with donepezil treatment was not paralleled by the pattern of improved task specific brain activation reported in similar studies of other ChEIs. This is arguably related to the different mechanisms of action of the ChEIs and might be a clinical correlate of the reported synaptic upregulation following long term donepezil treatment.