In this study, PDAPP transgenic animals were immunized with Abeta 42, in young animals (6 weeks) before the onset of AD-type neuropathologies and, older animals (11 months), when amyloid-beta deposition and several of the subsequent neuropathological changes are well established. In summary, data presented showed that outcomes of Abeta-plaque burden, neuritic dystrophy and gliosis were all significantly improved by Abeta 42 immunization in both young and old animals. Immunization preceding plaque development profoundly affected the occurrence of new lesions, as the progression of beta-amyloidosis and associated neuropathology was essentially wholly blocked. Amyloid-beta 42 immunization also significantly retarded the progression of existing pathology in affected regions of the older animals. Collectively, the results suggest that amyloid-beta immunization may prove beneficial for both the treatment and prevention of Alzheimer's disease.