Myotonic dystrophy (DM) is associated with an underlying CTG trinucleotide repeat expansion at a locus on chromosome 19q13.3. We have determined the repeat length in 23 DM patients with varying clinical severity of symptoms and various sizes of repeat amplification. We confirm that as in previous studies there is no strong correlation between repeat length and clinical symptoms but find that the repeat length in peripheral blood cells of patients increases over a time span of five years indicating continuing mitotic instability of the repeat throughout life. Repeat length progression does not appear to be indicative of clinical progression but age probably is. The degree of expansion correlates with the initial repeat size and 50% of the patients with continuing expansions showed clinical progression of their disease symptoms over the five year study period.