A wheelchair includes left and right side frames and a folding mechanism for connecting the left and right side frames to each other in a folding and unfolding relationship. The folding mechanism includes cross struts rotatably connected to the lower side frame members, hinge links connected to the upper side frame members, and a lock for locking the hinge links together when the folding mechanism is unfolded to form a rigid truss between the left and right upper side frame members.