Results of genealogical, DNA, and clinical findings in 41 families with 235 patients affected with myotonic dystrophy (DM) led to the following observations. (1) The relative proportion of affected patients among blacks is apparently lower than among whites or orientals. (2) A significant excess of males was observed. (3) The frequency of DM patients who did not reproduce was similar for males and females; however, female patients had on average 25% fewer children than male patients. (4) There was a significant intergenerational increase in the mean length of the CTG repeat which was also correlated with the severity of the phenotype. (5) No significant difference was observed in the mean size of the CTG repeat in offspring of male as compared to female transmitters. (6) With the exception of the congenital cases of maternal origin, the largest expansions were paternally inherited, but did not lead to congenital DM.