We have identified a hypervariable microsatellite sequence within the chromosome 9 marker MCT112 (D9S15), which we have previously shown to be tightly linked to Friedreich's ataxia (FRDA). The system detects 7 alleles ranging in size from 195 to 209 base pairs, and substantially increases informativity at the MCT112 locus. This enhances its use for genetic counselling in affected families. Recalculated combined linkage data between the FRDA locus and MCT112 gives a maximal lod score of 66.91 at a recombination fraction of theta = 0). There is no evidence of linkage disequilibrium.