The length of the CTG repeat in the 3' untranslated region of the DMPK gene is considered to be associated with clinical severity in type 1 Dystrofia Myotonica (DM1) and has also been suggested to correlate with the degree of deficiency of IgG noted in these patients. Total serum level of IgG and IgG subclasses was therefore measured in 61 Swedish patients with DM1, the largest number of patients investigated to date in this respect. Almost half (44%) of the DM1 patients showed a serum concentration of IgG below the normal range. Deficiency of IgG1, IgG2 or IgG3 was noted in 26%, 7% and 20% of the patients, respectively. As transcription of genes 3' of the DMPK gene on chromosome 19 is reduced in DM1 patients, a decreased expression of the alpha chain of the receptor involved in IgG catabolism, FcRn, may theoretically be responsible for the low serum IgG in DM1 patients. No correlation was however found between the number of CTG repeats, levels of FcRn transcripts in either muscle tissue or lymphocytes and serum IgG levels.