Abstract: An improved height adjustment mechanism for a chair seat back is disclosed. The improved height adjustment mechanism includes a U-shaped mounting bracket having a back wall and two side walls. A longitudinal cam having a handle is rotatably mounted in the bracket between the side walls and spaced from the back wall. A seat back support arm is insertable through the bracket between the cam and the bracket back wall. The cam is provided with a plurality of planar faces extending in the longitudinal direction disposed around about a third of the circumference of the cam. The radial length of the line extending perpendicularly from each planar face to the longitudinal axis of the cam increases steadily from one planar face to the next in one direction around the circumference of the cam. This steady increase in distance of the planar faces from the axis of the cam provides a cam action on the seat back support arm when the cam is rotated in one direction with the support arm located between the cam and back wall.