Five out of 10 patients with dystrophia myotonica showed a progressive ventricular enlargement on repeated pneumoencephalography. Enlargement occurred in all 3 patients with an interval between examinations of more than 5 yrs. Three patients with an interval of less than 2-1/2 yr. demonstrated no definite changes. These limited data suggest a slowly progressive cerebral degeneration during life in dystrophia myotonica. There appears to be a certain degree of correlation between the general clinical condition and the pneumoencephalographic findings. It is reasonable to assume that the cerebral as well as other organic changes are manifestations of a basic disorder which is of an undetermined nature but probably metabolic in origin.