Gate-all-around integrated circuit structures having nanowires with tight vertical spacing, and methods of fabricating gate-all-around integrated circuit structures having nanowires with tight vertical spacing, are described. For example, an integrated circuit structure includes a vertical arrangement of horizontal silicon nanowires. A vertical spacing between vertically adjacent silicon nanowires is less than 6 nanometers. A gate stack is around the vertical arrangement of horizontal silicon nanowires. A first source or drain structure is at a first end of the vertical arrangement of horizontal silicon nanowires, and a second epitaxial source or drain structure is at a second end of the vertical arrangement of horizontal silicon nanowires.