The theory of substantial performance arises with the purpose of preventing contractual resolution by default, without prior judicial consideration of the utility to the creditor of the termination of this agreement and the resulting damages to the debtor and third parties. The objective is to analyze the application of this theory by the Paraná Court of Justice (TJPR), specifically to the promise contracts of realty purchase and sale, and its possible socioeconomic repercussions. Based on a detailed research in the electronic base of jurisprudence of this Court, carried out on January 19, 2019, 121 decisions were found on this subject that, associated to the doctrinal analysis, supported this study. It was found that most of the decisions dealt with the possible maintenance of the contract on the basis of this theory only from the quantitative (mathematical) point of view, that is, if the payment percentage reached almost the end. The conclusion reached was that the lack of examination of the qualitative elements, related to the parties' behavior and the reason for the default, results in a superficial analysis, in which the contract is relativized without the integral evaluation of the elements, which causes legal uncertainty to the contractors, resulting in the price increase of the contracts and in the consequent adverse selection of the future promising buyers.