The initial observation of an expanded and unstable trinucleotide repeat in the Huntington's disease gene has now been confirmed and extended in 150 independent Huntington's disease families. HD chromosomes contained 37–86 repeat units, whereas normal chromosomes displayed 11–34 repeats. The HD repeat length was inversely correlated with the age of onset of the disorder. The HD repeat was unstable in more than 80% of meiotic transmissions showing both increases and decreases in size with the largest increases occurring in paternal transmissions. The targeting of spermatogenesis as a particular source of repeat instability is reflected in the repeat distribution of HD sperm DNA. The analysis of the length and instability of individual repeats in members of these families has profound implications for presymptomatic diagnosis. © 1993 Nature Publishing Group.