Amyloid beta protein (A-beta) deposition was investigated by quantitative immunohistochemistry in 13 cases of dementia with Lewy bodies (DLB) and compared with that in a series of age, gender and ApoE genotype matched cases of Alzheimer's disease (AD). In DLB the predominant A-beta peptide species deposited was A-beta-42(43) and this was similar in amount to that in AD. By contrast, A-beta(40) deposition was sparse in DLB and was lower than that in AD as was the total A-beta (A-beta-40 + A-beta-42(43) deposition. These data reinforce the viewpoint that in all disorders in which A-beta deposition is characteristics, the initial and predominant peptide species deposited is the longer form, A-beta-42(43). The density of Lewy bodies (LB) in DLB was unrelated to the extent of A-beta deposition, although those cases possessing one or more copies of the apolipoprotein E E4 allele had a higher LB density than those without an E4 allele. This suggests that the apolipoprotein E E4 isoform might facilitate, though not necessarily trigger, the formation of LB in susceptible individuals.