A folding chassis for push-chairs, baby carriages and the like, comprising - a lower frame, which has at least one front post and at least one rear post, which are connected in a lower region to respective wheel assemblies (3a, 3b); - at least one upper post, which supports grip means and can move with a combined rotary and translational motion with respect to said lower frame during the transition of said chassis from an extended open condition to a collapsed closed condition; - at least one interconnection linkage, which is articulated rotatably to said at least one upper post and to said at least one rear post about a first articulation axis and a second articulation axis; - means, comprising at least one upper traction element and at least one lower traction element, for kinematic interconnection between said at least one upper post and said lower frame; - said interconnection linkage and said kinematic interconnection means being adapted to determine the relative motion between said at least one front post, said at least one rear post and said at least one upper post during the transition of said chassis from the extended open condition to the collapsed closed condition and vice versa, - locking means which can be activated, as a consequence of the transition of said chassis from the collapsed closed condition to the extended open condition, and are adapted to keep said chassis in the extended open condition, - a deactivation device, which is adapted to deactivate on command said locking means in order to allow the transition of said chassis from said extended open condition to said collapsed closed condition, characterized in that at least one grip device comprises a handle that is movable, along a movement path, with respect to said lower frame, and at least one actuation device comprising a lever system which is connected to said handle and being adapted to operate by pushing against said interconnection linkage, said actuation device being able to be actuated, with said locking means deactivated and said handle of said grip device gripped by the user to apply an action to lift said lower frame by means of said handle of said grip device, in order to move, by means of a movement of said at least one linkage and of said at least one upper post, said first articulation axis away from said at least one rear post, so as to move beyond a position of equilibrium of said chassis beyond which said chassis tends to move towards said collapsed closed condition due to its own weight. - a lower frame, which has at least one front post and at least one rear post, which are connected in a lower region to respective wheel assemblies (3a, 3b); - at least one upper post, which supports grip means and can move with a combined rotary and translational motion with respect to said lower frame during the transition of said chassis from an extended open condition to a collapsed closed condition; - at least one interconnection linkage, which is articulated rotatably to said at least one upper post and to said at least one rear post about a first articulation axis and a second articulation axis; - means, comprising at least one upper traction element and at least one lower traction element, for kinematic interconnection between said at least one upper post and said lower frame; - said interconnection linkage and said kinematic interconnection means being adapted to determine the relative motion between said at least one front post, said at least one rear post and said at least one upper post during the transition of said chassis from the extended open condition to the collapsed closed condition and vice versa, - locking means which can be activated, as a consequence of the transition of said chassis from the collapsed closed condition to the extended open condition, and are adapted to keep said chassis in the extended open condition, - a deactivation device, which is adapted to deactivate on command said locking means in order to allow the transition of said chassis from said extended open condition to said collapsed closed condition,