The MASA syndrome is an X-linked disorder with mental retardation, spastic paraparesis, and adducted thumbs as the most characteristic features. We performed linkage analysis, using Xq28 markers, on a large MASA syndrome family. The maximum lodscore was 6.37 at 0 recombination for DXS52 and 5.99 at 0 recombination for DXS305. Crossovers were demonstrated between the disorder and DXS455. Clinical and linkage data from this family further support the hypothesis that the MASA syndrome and X-linked hydrocephalus are allelic disorders.