With aging, the content of self-reported autobiographical memories shifts from episodic to semantic. Onset of Alzheimer's disease enhances this pattern, but the neural underpinnings of this change in Autobiographical Memory (AM), in particular the role of hippocampal degradation, are unknown. We employed fMRI contrasting autobiographical and semantic retrieval, in 22 healthy elderly and 21 Alzheimer's patients. The shift towards semantic characteristics in AM retrieval was indeed enhanced in patients. Both groups activated brain regions commonly involved in AM retrieval, including occipital association areas, medial temporal lobes, lateral temporal and midline prefrontal areas. When compared to controls, Alzheimer's patients showed enhanced activity in the left inferior frontal gyrus (LIFG), ventromedial prefrontal cortex (vmPFC), right precuneus and left lingual gyrus. Activation of LIFG and vmPFC was significantly negatively correlated with hippocampal volume in patients only. Thus, we speculate that the linking function of the degraded hippocampus is taken over by the vmPFC; a shift recently observed during normal consolidation. This potentially compensatory process may support early Alzheimer's detection or prognosis.