Abstract: Adjusting mechanism for the step-wise height adjustment of the backrest of a work chair comprises two U-form guide bars embracing side edges of a rectangular support arm extending up from the seat of the chair. A C-form slide, on which the backrest is mounted, embraces and is slidable on the guide bars. One of the guide bars has a wider flange in which locking recess are formed. A locking lever pivotally mounted on the slide has a nose portion engageable in the locking recesses to hold the slide and hence the backrest in selected fixed position. The locking lever is spring biased to locking position and is releasable by a handle at the side of the slide so that the locking lever can be released and the slide, with the backrest, moved up or down with one hand.