A vehicle seat backrest assembly comprises a backrest pan, a rod in front of the pan, and a resilient sponge rubber pad overlying the rod and pan. Adjustable lumbar support means between the resilient pad and pan comprises: a support frame mounted on the rod, a manually operable adjustment lever pivotally connected to the support frame and extending across the frame and out through a slot in the backrest pan, a lumbar support member in the form of a generally rectangular resilient flexible forwardly convex grid having its lower edge connected to the support plate and having its upper edge connected to the lever, whereby raising or lowering of the lever causes a decrease or increase, respectively, in the convexity of the lumbar support member and the overlying pad. Interengageable bosses on the frame and the lever and biasing means maintain the lever in selected positions.