The discovery of an expanded (CTG)n repeat sequence in myotonic dystrophy (DM) has greatly improved our ability to detect DM gene carriers who have few or none of the classical signs of this disorder. We report here our experience with two such groups of gene carriers. We used a PCR based protocol that should be especially sensitive to small increases in CTG triplet number which might escape detection by conventional Southern blot analysis. Our analyses show that on 100 non-DM chromosomes the number of CTG triplets ranged from five to 37. We then studied 17 obligate gene carriers aged 55 years and over who showed no muscle weakness. All of the gene carriers in this group showed a relatively small increase in the number of CTG triplets (52 to 90 CTG triplets) with limited somatic mosaicism. We subsequently studied 11 subjects (aged 19 to 36 years) who had previously been identified as gene carriers by genetic linkage studies, but who lacked diagnostic signs. In this prospectively studied group, nine subjects showed an expanded allele, confirming the earlier prediction from linked genetic markers. The other two subjects had only two normal alleles and no expanded allele. Revision of the clinical data casts doubt on the original diagnosis of DM in their families. Preferential amplification of the normal non-expanded allele was noted in three asymptomatic gene carriers in this study (as well as in two of their clinically affected relatives). We caution that, at least in our hands, the DM mutation can be confidently excluded by this PCR based method only if both normal alleles have been identified.(ABSTRACT TRUNCATED AT 250 WORDS)