We describe three cases of early- (cases 1-3, 28-39 years) and one of late-onset (case 4, 76 years) Alzheimer's disease (AD) with 'cotton wool' plaques (CWPs) but without a family history indicating autosomal dominant inheritance. The early-onset cases, but not the late-onset case, showed remarkable aggression, disinhibition, and impulsiveness. Spastic paraparesis was observed in only one early-onset case. Hematoxylin-eosin-stained sections showed numerous CWPs, especially in the temporal cortex, in all cases. Bielschowsky-stained sections showed neurofibrillary tangles and minor neuritic changes surrounding the CWPs in three cases, but not in case 2. Gallyas-Braak-stained sections showed weak argyrophilia in homogeneous material of the CWPs in cases 2 and 4. Quantitative analysis demonstrated that Abeta42 was deposited more predominantly than Abeta40 in three cases. However, in case 2, approximately twice as much Abeta40 as Abeta42 was deposited. Tau immunostaining demonstrated neuritic changes in three cases, but not in case 2. alpha-Synuclein-positive Lewy bodies (LBs) and astrocytic lesions containing non-Abeta component of AD amyloid (NAC), a central fragment of alpha-synuclein, were found in case 3. In conclusion, (1) a frontal lobe syndrome-like personality change may be one of the characteristic clinical features of early-onset CWP-AD, (2) the deposition pattern of Abeta40 and Abeta42 in CWP-AD is more variable than that of presenilin-1-linked cases, (3) Abeta deposition can result in development of dementia without tau pathology, and (4) CWP-AD with LBs and several other neurodegenerative disorders with LBs share a common process involving alpha-synuclein and NAC deposition.