Abstract: A chair height adjustment mechanism selectively positions a seat section relative to a base and is situated between a barrel attached to the chair base and a threaded spindle extending downwardly from the seat section and telescopically received within the barrel. The adjustment mechanism includes an adjusting nut assembly receiving the spindle and extending between the barrel and spindle. A spring urges the nut assembly toward engagement with a detent member connected to the barrel. When the seat section is occupied, the spring is compressed and the nut disengages from the detent member, and the spindle and nut rotate together as the seat section swivels. When the seat section is unoccupied, the spring urges the nut into interlocking engagement with the detent member, and the spindle rotates relative to the nut as the seat section swivels in order to cause the seat section and spindle to adjust upwardly and downwardly depending on the direction of seat rotation relative to the base and barrel.