Linkage analysis and haplotype characterization for the allelic system detected at the 3' creatine kinase muscle type (CKMM) locus were carried out in 59 myotonic dystrophy (DM) families from Italy and Spain. A maximum lod score (zmax) of 21.26 at a recombination frequency (theta) of 0.00 was found. No statistically significant linkage disequilibrium was observed between DM and the RFLPs examined. However, a substantial linkage disequilibrium was found between CKMM-TaqI and CKMM-NcoI sites in these two populations.