Abstract: Assembly for a chair with adjustable seat back and leg-rest, which comprises a seat frame (14) secured at its two ends by hinges (12) to a seat back frame (13) and to a leg-rest frame (15) , these three frames (13-14-15) including in their lower portions connecting struts (16-25a-25b) positioned between one side and the other of the frames (13-14-15), the seat frame (14) comprising in its lower portion at least two reinforcing cross members (11) having the functions of resting on and being fastened to a basic characterizing structure (17) , the connecting strut (16) of the seat frame (14) bearing one single actuation means (18) that includes an actuation lever (21) which has a first inactive position and second (21a) and third (21b) positions to alter inclination, the second and third positions (21a-21b) to alter inclination cooperating with respective valves (23) which act on positioner jacks (20a-20b) located between the respective connecting struts (16-25a-25b).