The discovery that there is an expansion of a CTG repeat underlying myotonic dystrophy has led to new approaches in diagnosis and genetic counselling for this disorder. The size of the expansion correlates to a reasonable degree with the clinical symptoms within a given family. We report comparisons of the length of the expansion seen in lymphocytes and muscle samples from eight patients. In all cases the length of the expansion seen in DNA isolated from muscle was larger than that seen in lymphocytes from the same patient. There was no progression of the expansion over a period of 10-15 years in muscle samples from two of these patients even though there had been significant progression of the severity of symptoms during that time.