A 23-year-old man with Pelizaeus-Merzbacher disease had a novel mutation, C344A (Thr115Lys), in exon 3 of the proteolipid protein gene (PLP) His mother, heterozygous for the mutation, developed progressive personality change and a gait disorder in her mid-20s. Her MRI at age 53 showed a diffuse severe leukodystrophy. This report extends the phenotypic range of disease due to PLP gene mutations to include adult-onset dementia in females.