Abstract Objectives Expansion of CTG repeats in myotonic dystrophy (DM1) alters the regulated expression of numerous genes. It is considered to explain the major clinical features of DM1. IgG deficiency is common in DM1 and is due to altered FcRn-related hypercatabolism. We hypothesized that the IgG catabolic rate is correlated with CTG repeat expansion. Methods Correlations between serum immunoglobulin levels, peripheral lymphocyte subset counts and CTG repeat numbers were performed in 52 DM1 patients. Results Serum IgG and IgG1 levels were below the normal limit respectively in 54% and 72% of patients. Increasing CTG repeat numbers were significantly correlated with decreasing serum IgG and IgG1 levels, and with decreasing CD3+ T-cell and CD3+ –CD8+ cell counts. An abnormal immunoglobulin profile at protein electrophoresis was found in 4 patients. Conclusion We conclude that the catabolic rate of IgG is linked to expanded CTG repeats, possibly involving an altered immune response.