<i>Objectives:</i> To determine whether the mitochondrial DNA (mtDNA) A4336G mutation represents a risk factor for Spanish patients with both Alzheimer’s disease (AD) and Parkinson’s disease (PD). <i>Material and Methods:</i> One hundred and sixty-one AD and 106 PD unrelated patients were included in the study. Seventy-eight age-matched and 144 randomly chosen healthy subjects served as controls. The frequency of the A4336G mutation in these groups was compared using the χ² and Fisher’s exact tests. p < 0.05 was established as a statistically significant differential value. <i>Results: </i>The mtDNA A4336G mutation was present in 1/161 of AD patients (0.6%), in 3/106 of PD patients (2.8%), in 1/78 of age-matched controls (1.3%) and in 2/144 of the randomly chosen controls (1.4%). These differences were not statistically significant. <i>Conclusion:</i> Our results do not support the hypothesis that this mutation represents a risk factor for either AD or PD patients, at least in the case of this Spanish sample.