The spontaneous and mitomycin C (MMC) induced sister chromatid exchange (SCE) frequencies were examined in five adults with myotonic dystrophy (MD). There was no significant difference in the spontaneous incidence of SCE between MD patients and controls. However, a significantly enhanced response to the induction of SCE by MMC was observed in three of the five MD patients who were severely affected, while no such enhancement was observed in the other two mildly affected cases. These results suggest that the raised response may be a secondary consequence associated with disease progression. No differences were observed between the proliferating rate in vitro of lymphocytes of severe and mild cases of MD and controls, but the severe cases showed a significant increase in the proportion of T cells in peripheral blood. It is suggested that the observed enhanced response to MMC may reflect an increase in the proportion of a more sensitive subset of T cells in the peripheral blood.