Public scandals surrounding charitable nonprofit organizations have increased public scrutiny and criticism which resulted in an increased demand for accountability of charities and other nonprofit organizations. Reporting performance is necessary in order to be accountable for the organizations' performance. Performance reporting is more difficult in nonprofit organizations than in for-profit organizations since no profitability information can be communicated. Nonprofits tend to focus on other concepts to define performance; mainly efficiency, effectiveness and social value. The main theory on nonprofit performance is described in the Organizational Effectiveness literature. Impact assessment has been suggested as a way to measure nonprofit organizational performance. This concept is related to the effectiveness concept of goal attainment, but goes one step further and focuses on impact rather than mere outcomes. In this research a first attempt is made to measure the impact of a specific Dutch nonprofit organization, the Netherlands Heart Foundation. The impact is estimated on the costs related to AMI patients in the working population.