The deposition of abnormal protein fibrils is a prominent pathological feature of many different ‘protein conformational’ diseases, including some important neurodegenerative diseases such as Alzheimers disease (AD), Parkinsons disease (PD), motor neurone disease and the ‘prion’ dementias. Some of the fibril-forming proteins or peptides associated with these diseases have been shown to be toxic to cells in culture. A clear understanding of the molecular mechanisms responsible for this toxicity should shed light on the probable link between protein deposition and cell loss in these diseases. In the case of the β-amyloid (Aβ), which accumulates in the brain in AD, there is good evidence that the toxic mechanism involves the production of reactive oxygen species (ROS). By means of an electron spin resonance (ESR) spin-trapping method, we have shown recently that solutions of Aβ liberate readily detectable amounts of hydroxyl radicals upon incubation in vitro followed by the addition of small amounts of Fe(II). We have also obtained similar results with α-synuclein, which accumulates in Lewy bodies in PD. Our data suggest that hydrogen peroxide accumulates during Aβ or α-synuclein incubation and that this is subsequently converted to hydroxyl radicals, on addition of Fe (II), by Fentons reaction. Consequently, we now support the idea that one of the fundamental molecular mechanisms underlying the pathogenesis of cell death in AD, PD, and possibly some other protein conformational diseases, could be the direct production of ROS during formation of the abnormal protein aggregates. This hypothesis suggests a novel approach to the therapy of this group of diseases.