There is a large body of evidence that free radical-mediated oxidative damage is involved in the pathogenesis of neurodegenerative disease. Although it is unlikely that markers of such damage will have any diagnostic value, they might be of considerable interest in following disease progression and monitoring the efficacy of different treatments. Among such markers, there is evidence for the elevation of peripheral malondialdehyde levels in several neurodegenerative diseases, including Parkinson's disease, amyotrophic lateral sclerosis and Alzheimer's disease. The measurement of malondialdehyde levels, which is both simple and cheap to perform, can and should be incorporated into future clinical trials. This will allow a clearer picture to emerge as to whether malondialdehyde can be considered as a marker for the evolution of these diseases.