Projects become tools to support the goals of the organisation. A project portfolio is the set of all projects that are currently being implemented in an organization. Possible projects are characterized by sets of inputs and outputs, where inputs are the resources to implement the project and outputs measure multiple goals of the organization. Data envelopment analysis (DEA) is an appropriate approach to select individual effective projects. The contribution of this paper is to propose a model to measure the effectiveness of the entire project portfolio. An organization has its total resources in limited quantity. Designing a portfolio of efficient projects that does not exceed the limited resources does not always lead to the most efficient portfolio. This paper proposes a new approach to project portfolio design based on the use of an extended DEA model. The constraint on the project portfolio is the total available budget. The performance measure of the designed project portfolio is the efficiency of the portfolio and the effectiveness of the outputs. In practice, this approach results in achieving better outputs with lower inputs in terms of the overall project portfolio. Possible extensions to this approach are formulated and discussed.