We have investigated the pattern and extent of nucleotide diversity in 10 X-chromosomal genes where mutations are known to cause mental retardation in humans. For each gene, we sequenced the entire coding region from cDNA in humans, chimpanzees, and orangutans, as well as about 3 kb of genomic DNA in 20 humans sampled worldwide and in 10 chimpanzees representing two "subspecies." Overall nucleotide diversity in these genes is about twofold lower in humans than in chimpanzees, and nucleotide diversity within and between species is low, suggesting that a high level of functional constraint acts on these genes. Strikingly, we find that a summary of the allele frequency spectrum is significantly correlated in humans and chimpanzees, perhaps reflecting very similar levels of constraint at these genes in the two species. A possible exception is FMR2, which shows a higher number of nonsynonymous than synonymous substitutions on the human lineage, suggesting the action of positive selection.