Since a parental sex effect has been reported in Huntington's disease, we looked to see whether a similar effect is apparent in adult (autosomal dominant) familial ALS. We analyzed the data for 145 patients, with an onset age range of 20 to 68 years and a known affected parent (AP), from 52 families described in the literature. There was a significant increase in the percentage of patients inheriting the gene from an affected mother as a function of the age at onset. There was also a significant correlation between AP and offspring age at onset only when the AP was the mother.