In five families we have confirmed the close linkage between the genes for myotonic dystrophy and apolipoprotein CII. The total maximum lod (log of the odds) score was 3.32 at 0 recombination. We demonstrate that the use of a Ban I restriction site polymorphism for apolipoprotein CII adds additional useful information when combined with the more commonly used Taq I polymorphism. The potential practical clinical use of these markers for the prenatal diagnosis of myotonic dystrophy is demonstrated.