Voluntary industrial environmental management initiatives such as management systems based on the international standard ISO 14001has been widely spread over the world in recent years. Previous research on the influence of these systems relies heavily on how the benefits of the systems are perceived by companies’ representatives. Additionally, most studies tend to focus on the impact on the overall environmental performance of the companies, not identifying differences in different environmental aspects. The purpose of this paper is to explore the impact of voluntary environmental management systems on energy efficiency in Swedish manufacturing industry by means of objective industrial waste data derived from mandatory annual environmental reports. The study focuses on changes in energy efficiency over a period of 12 years and includes both ISO 14001-certified companies (66 companies) and non-certified firms (50 companies). Consideration is given to energy improvement efforts in the companies before ISO 14001 adoption. Analysis has been carried out using statistical methods for two different industrial energy parameters: electricity and fossil fuel consumption. The results indicate that ISO 14001 adoption has an impact on the energy efficiency regarding the use of fossil fuel. In contrast no impact has been found concerning the use of electricity.