Myotonic dystrophy(MyD) is caused by the abnormal expansion of CTG repeats in the 3' untranslated region of a gene encoding a protein kinase. Although some sporadic cases of MyD have been reported, the molecular basis of the intergenerational CTG expansion from asymptomatic parents is not fully understood. To determine the frequency of sporadic cases, we surveyed our clinical records of 40 MyD patients. Four cases with no family history were found, and we analyzed DNA extracted from the peripheral-blood lymphocytes of 2 unrelated patients and their family members. We identified 3 asymptomatic cases, 2 male and 1 female, with premutations which were genetically defined as 40 to 50 abnormal repeats of CTG. The fact that, in our study, the transmitting parents were both male may indicate that paternal transmission could be significant in sporadic MyD cases. We also reviewed the literature of premutation in triplet diseases and discussed the molecular mechanism of marked elongation in triplet repeats during transmission.