36 relatives of 2 patients with myotonic dystrophy (M.d.) were investigated during a field study. All were neurologically examined at home and their ABO blood groups and ABH-secretor phenotypes in parotid saliva were determined for linkage analyses. Additional investigations such as EMG or slit-lamp examinations were carried out by specialists in doubtful cases. Nine additional patients presenting with varying clinical manifestations of M.d. were found. In most of the secretor-positive individuals the corresponding genotypes could be identified as heterozygotes (Sese) through the segregation pattern of their first-degree relatives. In both families the allele for M.d. was linked to the dominant allele Se, the probability being greater than 99%. A recombination was observed in 1 patient of either family. This is, however, fairly consistent with the expected recombination fraction of 0.08. No further linkage information was obtained from determinations of the Lutheran blood groups carried out in family no. 1 only, since all individuals were of the most frequent type Lu (a-/b+). In 5 out of 8 matings between M.d. patients and healthy spouses the secretor genoypes were potentially informative for the estimation of risk of their children being affected or for prenatal diagnoses. The secretor analyses and special tests for the detection of preclinical cases (e.g. by slit-lamp) and their practical value for genetic counseling are discussed, especially with regard to the limitations, which derive partly from low probabilities in the predictions even in some of the so-called informative matings, and partly also from the recombinations to be expected.