Protein misfolding and aggregation have been linked to several human diseases, including Alzheimer's disease, Parkinson's disease, and systemic amyloidosis, by mechanisms that are not yet completely understood. The hallmark of most of these diseases is the formation of highly ordered and beta-sheet-rich aggregates referred to as amyloid fibrils. Fibril formation by WT transthyretin (TTR) or TTR variants has been linked to the etiology of systemic amyloidosis and familial amyloid polyneuropathy, respectively. Similarly, amyloid fibril formation by alpha-synuclein (alpha-syn) has been linked to neurodegeneration in Parkinson's disease, a movement disorder characterized by selective degeneration of dopaminergic neurons in the substantia nigra. Here we show that consecutive cycles of compression-decompression under aggregating conditions lead to reversible dissociation of TTR and alpha-syn fibrils. The high sensitivity of amyloid fibrils toward high hydrostatic pressure (HHP) indicates the existence of packing defects in the fibril core. In addition, through the use of HHP we are able to detect differences in stability between fibrils formed from WT TTR and the familial amyloidotic polyneuropathy-associated variant V30M. The fibrils formed by WT alpha-syn were less susceptible to pressure denaturation than the Parkinson's disease-linked variants, A30P and A53T. This finding implies that fibrils of alpha-syn formed from the variants would be more easily dissolved into small oligomers by the cellular machinery. This result has physiological importance in light of the current view that the pathogenic species are the small aggregates rather the mature fibrils. Finally, the HHP-induced formation of fibrils from TTR is relatively fast (approximately 60 min), a quality that allows screening of antiamyloidogenic drugs.