A Lighting device for vehicles, such as a headlight or a headlamp or rear lamp, comprising a housing designed to be mounted on a vehicle and carrying at least a light source and at least one light guide, the housing having a front face in use facing opposite to the vehicle and along a driving direction, the light guide being designed to collect light rays from the light source and to deliver the light rays collected from the light source along an optical axis transverse to said front face and extending in the driving direction; the light guide being shaped as an elongated element made of a material pervious to light having a first and second opposite ends (9,10) and a solid cross section delimited by lateral surfaces (13,14;15,16) designed to have total reflection capacity so that the light rays collected at one end are guided towards the other end and vice versa; at least a first stretch of the elongated element constituting the light guide being provided on at least a rear lateral surface thereof with light deviating means to direct the light rays travelling along the light guide against a front lateral surface of the light guide, facing the same direction of the front face of the housing, with an angle of incidence that allows the light rays to pass through the front lateral surface of the light guide to be emitted from the light guide along the optical axis, the light guide being arranged transversally to the optical axis and parallel to the front face of the housing; wherein - at least the first stretch of the elongated element constituting the light guide has its cross section variable in shape along a direction of travelling of the light rays along the light guide; characterized in that i)- the elongated element constituting the light guide receive the light rays from the light source at its first end, which is arranged far away from the front face of the housing and flattens progressively towards its second end at said first stretch thereof; and ii)- the elongated element constituting the light guide comprises said first stretch, which is arranged towards the second end, and a second stretch arranged towards the first end; the second stretch having a substantially constant cross-section of a symmetrical shape; and the first stretch being integrally provided with locking means to the housing. - at least the first stretch of the elongated element constituting the light guide has its cross section variable in shape along a direction of travelling of the light rays along the light guide; characterized in that i)- the elongated element constituting the light guide receive the light rays from the light source at its first end, which is arranged far away from the front face of the housing and flattens progressively towards its second end at said first stretch thereof; and ii)- the elongated element constituting the light guide comprises said first stretch, which is arranged towards the second end, and a second stretch arranged towards the first end; the second stretch having a substantially constant cross-section of a symmetrical shape; and the first stretch being integrally provided with locking means to the housing. i)- the elongated element constituting the light guide receive the light rays from the light source at its first end, which is arranged far away from the front face of the housing and flattens progressively towards its second end at said first stretch thereof; and ii)- the elongated element constituting the light guide comprises said first stretch, which is arranged towards the second end, and a second stretch arranged towards the first end; the second stretch having a substantially constant cross-section of a symmetrical shape; and the first stretch being integrally provided with locking means to the housing.