The enterprise-wide Information Technology (IT) platform at transmission network owners and electric utilities has normally been gradually developed for a long period of time. Each business unit within these companies have, unfortunately often independently, developed and acquired the IT systems needed. The IT platform is therefore normally composed of a considerable number of components, storing redundant data and implementing similar functionality. In the re-regulated power industry, the need for management of IT is made important by the frequent changes in the business environment, such as mergers, acquisitions, and new market rules. IT governance denotes how the IT organization is managed and structured and provides mechanisms that enable the development of integrated business and IT plans, allocation of responsibilities within the IT organization, and prioritization of IT initiatives. It is important to ensure that IT governance is designed not only to achieve internal efficiency in the IT organization, such as deploying good IT processes. The final goal of good IT governance is rather to provide the business with the support needed in order to conduct business in a good manner. Of particular interest is the impact on business performance that a well governed IT organization can have. The Control Objectives for Information and related Technology (CobiT) framework provides the most relevant and detailed support for IT governance processes, activities, roles, documents and metrics available. CobiT?s entities are mainly of desccriptive nature and propose how the IT organization should be structured according to best practice. Another inherent weakness is that CobiT focuses on entities inherent to the IT organization and do not address the external effectiveness of the IT organization. This article desccribes the IT Organization Modeling and Assessment Tool (ITOMAT) which is an IT governance decision support tool. It relies partly on the CobiT framework for internal maturity assessments of the IT organization and is also enhanced with information about statistical correlations to external IT governance performance metrics. This means that the ITOMAT is able to statistically predict external business effectiveness of suggested IT organization scenarios. The paper also presents an example in which ITOMAT is used to predict IT governance performance. The paper ends with discussion, conclusions and references.