The invention relates to a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterised in that it comprises one or more compounds of the formula I in a total concentration of ≥40%, in which the parameters have the meanings indicated in claim 1, and to the use of the liquid-crystalline medium for electro-optical purposes, in particular in liquid-crystal light valves for lighting devices for motor vehicles, and to liquid-crystal light valves which comprise this medium and lighting devices for motor vehicles which contain such liquid-crystal light valves.