Along with the oil price, concerns about the security of energy supply have soared once again in recent years. Yet, more than 30 years after the OPEC oil embargo in 1973, energy security still remains a diffuse concept. This article conceives a statistical indicator that aims at characterizing the long-term energy supply risk of nations that are heavily dependent on energy imports. Our indicator condenses the bulk of empirical information on the imports of fossil fuels originating from a multitude of export countries as well as data on the indigenous contribution to the domestic energy supply, thereby providing us with a single parameter. Applying the proposed concept to empirical energy data on Germany (1980-2004), we find that the energy supply risk has increased substantially since the 1970s. This outcome is mainly due to the drastic raise of oil and gas imports from Russia.