The first aim of this work is to identify New Institutional Economics influence upon judicial justification of the sentences involving a very important case happened in Brazil: the breach of automobile leasing contracts indexed to dollar (what begun by the end of government’s control over international currency and a great demonetization of national currency, in 1999). The second aim was not intentionally wanted during the first work project, but became essential part of the job because of the several information discovered after researches made during the investigations for the first aim. It identifies defaults of New Institutional Economics in explaining what empirically happened in the case studied. The first aim is developed through the theoretical construction of a judge and a mediator that represent New Institutional Economics – Judge Douglass and Mediator Oliver. Through the analyses of more than 6,000 decisions, it remained clear that judges helped themselves of economic theory to justify their decisions. The second aim of this work is based on the inability of prediction of the New Institutional Economics, specifically in this case. This is shown by testimonials of leasing dealers and the statistical analysis that insinuate the lack of correlation between the content of judicial decisions and the behave of automobile leasing market, a quite surprising discover concerning New Institutional Economics.