Abstract: A lumbar support for seat backs, in particular of motor vehicle seats, has a forward-bowed support plate (1). The support plate (1) is height-adjustable, moving upward and downward on guide organs that are fixed with respect to the seat back, and is also variable in its curvature through the use of a motor-driven tension bracket that can be lengthened or shortened. The tension bracket consists of bracket regions (3a and 3b) that act on upper end region (1a) and lower end region (1b) of the support plate (1). The bracket regions have thread regions of opposite handedness pointing toward one another which are externally engaged by a common rotatable sleeve (11) with appropriately designed meshing areas. The drive motor (10) is fixed to a gear case (2) arranged behind the support plate (1). The motor (10) drives gear wheels of a gearbox whose final gear wheel (13a) stands in engagement with the sleeve (11). The sleeve has inside thread regions of opposite handedness.