Introduction: Plasma Ab levels have been examined in sporadic Alzheimer's disease (AD) patients yielding conflicting results. Some studies showed no difference between plasma concentrations of A beta(1-42) and A beta (1-40) in sporadic cases of AD as compared to controls, others found increased levels of A beta(1-42) in at least some AD patients. The results of several recent studies suggest that elevated plasma A beta(1-42) levels may be detected several years before the onset of symptoms, though the value of that effect in predicting progression to dementia in mildly cognitively impaired (MCI) subjects is not known. Finally, it's been proposed that plasma Ab levels increase merely with age and are neither sensitive nor specific for AD or MCI. Material and method: Levels of A beta(1-40) and A beta(1-42) were measured in plasma from 54 patients with AD, 39 subjects with MCI and 35 controls using a commercially available ELISA. Results: Mean plasma A beta(1-42) levels were significantly higher in MCI as compared to both AD (p<0.001) and control subjects (p<0.001) while levels of A beta(1-40) did not differ between the groups. In contrast to some earlier reports no correlations were observed between A beta species levels and age or MMSE scores. However, A beta(1-42) were significantly lower in subjects carrying at least one apoliprotein epsilon 4 allele. Employing ROC curve analysis we found that the maximum accuracy in discriminating MCI versus both controls and AD subjects has been achieved using a cut-off value of 3.8. Conclusions: Mean plasma levels of Ab peptides differ between AD, MCI and control subjects though their usefulness in the differential diagnosis of AD is doubtful. Further studies are needed to establish the value of Ab peptides levels in identifying patients with MCI and (possibly) in prediction of their progression to clinically overt AD.