We report a 54-year-old man who was admitted to the hospital because of acute neurological symptoms due to a cerebral haemorrhage. Postmortem brain examination revealed a lobar haemorrhage and advanced AD neuropathologic changes associated with severe cerebral amyloid angiopathy. Genetic study evidenced the presence of a large APP locus duplication (APPdup) in the patient and a PSEN1 p.E318G polymorphism in him and his older asymptomatic sibling. The APPdup spanned 14.5 Mb and blocks of segmental duplications were detected in the breakpoints. We propose the replication-based mechanism of Fork Stalling Template Switching (FoSTeS) to explain this APPdup rearrangement.