The specific mutation in Huntington's disease (HD) is an expansion of the unstable CAG trinucleotide repeat in the IT 15 gene in chromosome 4p. We examined the relationship between the CAG repeat size and clinical presentation in 36 patients with suspected diagnosis of HD. Twelve patients had no relatives with documented HD, and five of them failed to show the expanded (>37) CAG repeats. The remaining 31 patients, including seven patients with atypical clinical features for HD (three without and four with family history of documented HD), were heterozygotes for the CAG repeat expansion. There were large CAG repeats (50 copies) in paternally transmitted HD cases with early onset (age 30 or earlier). The rate of disease progression was faster in paternally transmitted cases regardless of the CAG repeat length or age of onset. We conclude that (1) patients lacking the family history of HD frequently show no expansion of the CAG repeats, and (2) the sex of the affected parent influences both the CAG repeat size and the phenotypic expression of the HD gene in the offspring.