We investigated the proteolipid protein (PLP) gene in two brothers in a Japanese family with a connatal form of Pelizaeus-Merzbacher disease (PMD). Direct sequencing of the PLP gene revealed an A-to-T transition in exon 4, which led to an Asp-to-Val substitution at residue 202. Their mother was confirmed to be heterozygous for the mutation. The mutation was not found in 78 X-chromosomes of normal Japanese individuals. A correlation between the clinical severity of the disease in the brothers and the Asp202-to-Val mutation in the PLP gene was suggested.