Myotonic dystrophy type 1 (DM1) is a result of a CTG expansion in the 3'-untranslated region of the DMPK gene. DM1 is rare among African blacks who have fewer large CTG repeats in the normal range than other racial/ethnic groups. Neither the prevalence of DM1 nor the relationship of CTG expansion to clinical status in African Americans (AAs) is well documented. We describe two AA brothers with DM1, each of whom had CTG repeats of 5/639; their father was reported to have DM1 and had CTG repeats of 5/60. Other family members had CTG repeats of 5-14. An unrelated AA patient from a second kinship also had DM1; an analysis revealed CTG repeats of 27/191. In 161 Alabama AA control subjects, we observed 18 CTG alleles from 5 to 28 repeats; the most common allele had five CTG repeats. The frequency of CTG repeats >or=15 were greater (p < 0.0003) in Pygmy, Amhara Ethiopian, Ashkenazi Jewish, North African Jewish, Israeli Muslim Arab, European white, and Japanese populations than in the Alabama AA population. These data suggest that the risk for DM1 in AAs is intermediate between that of African blacks and whites of European descent.