In Reply We thank Williams and Malm for their interest in our Viewpoint1 on structural brain changes in astronauts, which we have termed hydrocephalus associated with long-term spaceflight (HALS). As Williams and Malm indicate, we,2 along with others,3 have documented an 11% to 14% increase (or approximately a mean [SD] increase of 2.9 [3.0] mL) in ventricular volume following long-duration spaceflights of 6 months.