Sueyoshi et al. (2009) have examined a synergy effect between electricity and gas services in the US electric utility industry. They have compared electricity-specialized firms with diversified utility firms in their financial performance and corporate value. A problem of their study is that it has not empirically measured the operational performance of the electric utility firms. As an extension of the preceding study, this research investigates the operational performance of 104 US electric utility firms (1990-2004) by fully utilizing DEA (Data Envelopment Analysis). This study finds the three new policy implications. First, the synergy effect has not existed in the operational performance of diversified utility firms before and after the deregulation on the US electricity markets. Thus, core business concentration is more effective for electric utility firms than corporate diversification to enhance their operational performance under the current US deregulation policy. Second, the operational performance has had an increasing trend until 1996 and a decreasing trend after 1996. Thus, the US deregulation policy has been influential on their operational performance. Third, the enhancement in operational performance of electric utility firms has improved their financial performance. The improvement in financial performance has increased their corporate value. Thus, this study finds the business causality among operational performance, financial performance and corporate value in the US electric utility industry. (C) 2010 Elsevier Ltd. All rights reserved.