CAG repeat expansions have been identified as the disease-causing dynamic mutations in the coding regions of genes in several dominantly inherited neurodegenerative disorders, including spinobulbar muscular atrophy, Huntington's disease, dentatorubral-pallidoluysian atrophy, spinocerebellar ataxia type 1, 2 and 6 and Machado-Joseph disease. The CAG repeat expansions are translated to elongated polyglutamine tracts and an increased size of the polyglutamine tract correlates with anticipation, the cardinal feature, seen in all these diseases. Autosomal dominant pure spastic peraplegia (ADPSP) is a degenerative disorder of the central motor system clinically characterized by slowly progressive and unremitting spasticity of the legs, hyperreflexia and Babinski's sign. Like the established CAG repeat diseases ADPSP is characterized by both inter- and intrafamilial variation and anticipation. Using the Repeat Expansion Detection (RED) method, we have analyzed 21 affected individuals from six Danish families with the disease linked to chromosome 2p21-p24. We found that 20 of 21 affected individuals showed CAG repeat expansions versus two of 21 healthy spouses, demonstrating a strongly statistically significant association between the occurrence of the repeat expansion and the disease (Fisher's test, P < 10(-5)) suggesting that a CAG repeat expansion is involved presumably as a dynamic mutation in ADPSP linked to chromosome 2p21-p24. The size of the expansion is estimated to be > or = 60 CAG repeat copies in the affected individuals. The CAG repeat expansion is very likely translated and expressed as indicated by the detection of a polyglutamine-containing protein in an ADPSP patient.