We investigated the relationship between the most common 4977-bp deleted mitochondrial DNA (mtDNA) mutations and the occurrence of sporadic amyotrophic lateral sclerosis (ALS). Primer-shift and quantitative polymerase chain reaction (PCR) were used to determine the 4977-bp deleted mtDNA in the muscle specimens from 36 patients with sporadic ALS and 69 age-matched controls with other neuromuscular disorders. We found that the 4977-bp deleted mtDNA mutations were significantly higher in the ALS patients than controls in both frequency (50.0% vs. 8.7%, P < 0.01) and amount (0.35 +/- 0.53% vs. 0.085 +/- 0.35%, P < 0.05). Subjects with, rather than without, deleted mtDNA were at a significantly higher risk for having ALS after adjustment for age and sex. Moreover, male subjects had a higher risk than female subjects of having sporadic ALS. This study suggested that 4977-bp deleted mtDNA is significantly associated with the occurrence of sporadic ALS.