Two elderly siblings with myotonic dystrophy (MD) were admitted with symptoms and signs of normal pressure hydrocephalus, the diagnosis being confirmed by CT scan and infusion test. A further 11 younger members of the family also had MD; in 2 of these, none of whom had cerebral symptoms, CT scan showed cerebral ventricular enlargement. Based on these findings, together with earlier reports on progressive cerebral ventricular enlargement in patients with MD, it is suggested that in some cases normal pressure hydrocephalus may be a late complication of MD.