A lighting unit (1) preferably providing backlighting for a flat display device (2) and operating substantially in a vertical working position. The lighting unit (1) includes lamps (3), which are in thermal contact with a heat-conducting wall (5) of a hollow box (6). The lamps (3) make thermal contact with the heat-conducting wall (5) on the side opposite the light-emitting side of the lighting unit (1). The hollow box (6) is provided with a rear wall (8) that runs essentially parallel to the heat-conducting wall (5) and has openings (9) which are distributed across the area thereof. A top sidewall (10) and a bottom sidewall (11) are provided with additional openings (12, 13) in zones that lie closer to the rear wall (8). Preferably, the sidewalls do not have openings in zones located closer to the heat-conducting wall (5) than the additional openings (12, 13).