We examined (CTG)n lengths in various tissues from a 70-year-old man with myotonic dystrophy type 1 (DM 1) who had a small 60-70 (CTG), expansion in his leukocytes. He died of renal cell carcinoma 5 years after a total laryngectomy for laryngeal carcinoma. Southern blot and polymerase chain reaction analyses were done on tissues obtained at autopsy. In the various normal tissues, (CTG). lengths were almost all the same size, whereas the renal cell carcinoma and metastatic tissues had longer lengths. When compared with the lengths in leukocytes about 5 years previously, (CTG)n lengths in the normal tissues were the same size. These findings suggest that both somatic instability and age-dependent (CTG). expansion in DM 1 patients with a small expansion may be less dominant than in patients with large expansions.