Huntington's disease (HD) is associated with an expanded unstable (CAG)n repeat in the IT15 gene. This repeat was investigated in 44 HD patients and 59 of their relatives at risk who were members of 29 unrelated families from various parts of Greece. Abnormal elongation of the (CAG)n repeat ranging from 39 to 95 trinucleotide units was found in all but one of the 44 HD patients tested with 70% of these patients showing 42-47 repeats. The size of the expanded sequence correlated inversely with the age at disease onset (r = 0.77, p < 0.00001, n = 43). In a single sporadic case, de novo expansion of the (CAG)n repeat was detected. Twenty-four of 59 asymptomatic family members at risk showed expansion of the (CAG)n repeat in the HD range (39-56 trinucleotide units) while three had intermediate alleles (36-37 repeats). Evaluation of the adjacent polymorphic (CCG)n repeat showed a strong linkage disequilibrium between the 7-unit (CCG)n repeat allele and the HD mutation, with 51% of normal and 93% of HD chromosomes showing this allele (chi 2 = 15.55, p < 0.0001, n - 260). These data on HD patients of Greek origin are consistent with the thesis that the (CAG)n expansion is the primary gene defect of the disease and that this mutation occurred primarily on chromosomes with the (CCG)7 repeat haplotype.