The genetic basis of myotonic dystrophy is an unstable expansion of CTG repeats located in a gene on chromosome 19 that encodes a putative serine/threonine protein kinase. We studied the somatic mosaicism of the (CTG)n expansion in myotonic dystrophy patients. (CTG)n expansions were 2- to 13-fold greater in DNA isolated from skeletal muscle than in DNA from leukocytes in 10 of 11 patients with myotonic dystrophy. Different muscles of the same individual showed similar (CTG)n expansions. In postmortem tissues from an adult patient, (CTG)n expansions in brain, skeletal muscle, cardiac muscle, testes, and liver were all greater than in leukocytes. Normal myotonic dystrophy gene alleles from 7 healthy subjects had the same number of CTG repeats in leukocytes and muscle. The myotonic dystrophy mutation displays pronounced heterogeneity in somatic cells. The (CTG)n expansion observed in peripheral blood leukocytes is not necessarily representative of the repeat expansion in affected tissues, such as skeletal muscle and myocardium. In some patients with myotonic dystrophy, the predictive value of genetic analysis based on leukocyte DNA may be limited.