The use of daytime running lights was made mandatory for new cars in Norway in 1985 and for all cars in 1988. This paper examines the effectiveness of this regulation as an accident countermeasure. The paper relies on the same study design and method of analysis as previous studies of similar laws in Finland and Sweden. Four hypotheses concerning the effects of daytime running lights are tested. None of them was supported. The total number of multiparty accidents in daylight was not reduced. Pedestrian accidents and accidents in twilight were not reduced. The number of rear-end collisions increased by about 20%. Daytime running lights appear to reduce daytime multiparty accidents only during summer (by about 15%) and only for multivehicle accidents, excluding rear-end collisions. The possibility that confounding factors may have influenced study results is examined. It is concluded that such an influence cannot be ruled out. The discussion of the results highlights the difficulties of reaching clear and defensible conclusions in nonexperimental accident research of the kind reported in this paper.