An active role of monoamine oxidase B (MAO-B) in the pathogenesis of neurodegenerative disorders such as Parkinson's disease has been proposed as the enzyme is known to be a generator of free radicals which seem to be responsible for neuron oxidative damage. We evaluated the influence of MAO-B in the pathogenesis of the sporadic forms of Amyotrophic lateral sclerosis (ALS) by studying the MAO-B allele distribution in 51 patients and 71 healthy controls. MAO-B did not directly result in a risk factor for ALS but seemed to strongly influence age at onset. The mean ALS onset age was significantly higher in individuals carrying allele 5 compared to individuals without this allele (60.4 +/- 8.1 vs. 52.1 +/- 10.3 years; P = 0.004). These results, in agreement with findings in the literature, suggest an increased MAO-B expression in ALS and support the hypothesis that neuronal cell death in neurodegenerative diseases is triggered by astroglial reaction.