Myotonic dystrophy (DM) is associated with an increased number of CTG repeats in the 3' untranslated region of the myotonin gene. Because DM has been observed less frequently in Ashkenazic Jews and non-Jews than in North African and Yemenite Jews in Israel, a study of the CTG repeat polymorphism was undertaken in these four groups. Alleles from 126 unrelated healthy North African Jews, 103 Yemenite Jews, 103 Ashkenazic Jews, and 106 Israeli Moslem Arabs were studied by PCR analysis of the trinucleotide repeat in the DM gene, and the size distribution of the CTG repeat was determined. The alleles ranged in length from 5-28 repeats in the Yemenite Jews, 5-26 in Muslim Arabs and North African Jews, and 5-23 in the Ashkenazic Jews. North African and Yemenite Jews were found to have significantly more large repeats in the normal range than Ashkenazic Jews and Muslim Arabs (for over 18 repeats: 9.1% and 13%, respectively, compared to 2.4% and 3.3%, respectively; P < 0.0001). It is suggested that the more frequent occurrence of large CTG repeats in the normal range may represent a greater predisposition to DM.