We report on a myotonic dystrophy (DM) family exhibiting instability of normal sized (CTG)n alleles in the DM kinase gene on the non-DM chromosome. At least two mutational events involving normal DM alleles must have occurred in this family; one was characterised as a 34-35 (CTG)n repeat mutation. These findings represent a dissociation between (CTG)n repeat instability and myotonic dystrophy. Furthermore, this family highlights genetic counselling issues relating to the pathogenicity of alleles at the upper end of the normal size range and the risk of further expansion into the disease range.