Myotonic dystrophy is associated with an increased number of CTG repeats in the 3' untranslated region of the myotonin protein kinase gene. The recent elucidation of the molecular basis of myotonic dystrophy has, for the first time, made a specific molecular diagnosis of this condition a possibility. Ten South African families were analysed at the molecular level, using both PCR and Southern blot analyses for the detection of the trinucleotide repeat. Expansion of this repeat was found in 9 families and in 2 cases the grandparental origin, which was previously unknown, could be determined. It is now possible to counsel these families more effectively and to identify individuals, particularly women, who are at risk of passing on the DM mutation.