A carriage unit for sliding doors or windows of the lift and slide type, the door or window comprising at least one fixed frame and a sash horizontally slidable relative to the frame, between an open position and a closed position; said carriage unit comprising at least one pair of main and secondary carriages (4, 5), associated, in use, with the bottom crosspiece of the sash and resting on a horizontal base track to allow the sliding of the sash in both the opening and closing directions; the pair of carriages (4, 5) being joined together, in use, by a rigid connecting unit; at least the secondary carriage comprising: - a first box-type frame having an upper portion configured to be coupled, in use, to the lower crosspiece of the sash; - a second frame supporting a pair of wheels (10, 11) positioned, in use, resting on the track; the second frame being configured to allow a pivoting movement of the two wheels (10, 11) about a single pivot point; - a link mechanism for lifting and lowering the sash relative to the track, corresponding to the open and closed positions, respectively, of the sash; the link mechanism being positioned and acting between the first frame and, in use, the rigid connecting unit with the main carriage; - adjustment means for adjusting the distance of the movable sash relative to the track at least in its lowered position, positioned between the first frame and the second frame and acting directly on the first frame; where the adjustment means comprise: - the second frame equipped with a pair of slots with a vertical extension engaged in the single pivot point of the wheels (10, 11); - a third frame connected to the second frame at the single pivot point using a pair of inclined slots; - movement means positioned on the secondary carriage and designed for translating the third frame in both directions with corresponding lifting and lowering of the second frame along the single pivot point with relative lifting and lowering of the first frame and of the sash relative to the track; characterised in that the link mechanism comprises two connecting rods (12a, 12b) having only two pivot points each (P4, P2; P3, P6) respectively in the front and in the rear; in that the third frame is interposed between the pair of wheels (10, 11) and the second frame; and in that the movement means comprise a block slidably housed inside the first frame and configured for connecting to the rear ends of the second and third frame and moving the rear ends of the second and third frame, and adjustable means for adjusting the height of the sash, housed in the block, and in contact with the link mechanism forming a fixed contrast point when adjusting the height. - a first box-type frame having an upper portion configured to be coupled, in use, to the lower crosspiece of the sash; - a second frame supporting a pair of wheels (10, 11) positioned, in use, resting on the track; the second frame being configured to allow a pivoting movement of the two wheels (10, 11) about a single pivot point; - a link mechanism for lifting and lowering the sash relative to the track, corresponding to the open and closed positions, respectively, of the sash; the link mechanism being positioned and acting between the first frame and, in use, the rigid connecting unit with the main carriage; - adjustment means for adjusting the distance of the movable sash relative to the track at least in its lowered position, positioned between the first frame and the second frame and acting directly on the first frame; - the second frame equipped with a pair of slots with a vertical extension engaged in the single pivot point of the wheels (10, 11); - a third frame connected to the second frame at the single pivot point using a pair of inclined slots; - movement means positioned on the secondary carriage and designed for translating the third frame in both directions with corresponding lifting and lowering of the second frame along the single pivot point with relative lifting and lowering of the first frame and of the sash relative to the track; characterised in that the link mechanism comprises two connecting rods (12a, 12b) having only two pivot points each (P4, P2; P3, P6) respectively in the front and in the rear; in that the third frame is interposed between the pair of wheels (10, 11) and the second frame; and in that the movement means comprise a block slidably housed inside the first frame and configured for connecting to the rear ends of the second and third frame and moving the rear ends of the second and third frame, and adjustable means for adjusting the height of the sash, housed in the block, and in contact with the link mechanism forming a fixed contrast point when adjusting the height.