Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disorder characterized by marked variability of its clinical manifestations. The mutational basis of DM is an unstable (CTG)n trinucleotide repeat in the 3' untranslated region of the myotonic dystrophy protein kinase gene (DMPK). We used quantitative RT-PCR to determine DMPK mRNA levels in muscular biopsies from three congenitally affected (CDM) and two control infants. The CDM infants had increased DMPK mRNA levels, which were not correlated to increased expression of the mutant allele. This increase may be the consequence of a maturational muscular arrest, which may maintain an elevated level of DMPK mRNA until birth.