Background: Many patients with myotonic dystrophy type 1 (DM1) develop muscular atrophy and weakness accompanied with myotonia in their adulthood. Some patients with DM1 have cataracts and heart problems without muscular weakness. CTG expansion in 3' untranslated region of DMPK gene causes DM1. The detection of this mutation has been done using Southern blotting or PCR-Southern blotting. But these methods need many procedures. Recently, the triplet repeat primed PCR (TP-PCR) was developed to detect some nucleotides repeat expansions. Objective: We tried to apply this TP-PCR methods to detect the Japanese DM1 patients and aimed to evaluate the efficacy for screening DM1. Patients and methods: We took peripheral blood from 10 Japanese patients in our hospital with informed consent. Three of them had cataracts and abnormal ECG findings without muscular weakness. We designed primers and an anchor-primer according to the previous reports about TP-PCR for SCA36 and C9ORF72. We examined the TP-PCR and fragment analysis each with fluorescent-labeled primers by autosequencer, and did PCR-Southern blotting analysis. Results: The PCR-fragment analysis revealed mild expansion within 100 repeats in three patients without muscular weakness. Seven other patients had smear-like abnormal band in the PCR-Southern blotting. TP-PCR showed smear-like-peaks in all 10 patients including three mild ones. Conclusion: This method could detect all abnormal expansions. But above 100 repeats, all patients showed almost the same pattern and could not show the difference of the size of repeats. The triplet repeat primed PCR is a convenient and useful method for screening DM1.