Although foreign to the Civil Code, the concept of opportunity cost is often seen in judicial or arbitral proceedings, in connection with indemnity claims brought up by the parties. While some studies of Civil Law mention the concept, directly or indirectly, it is necessary to analyze it scientifically from the perspective of its compatibility with the Brazilian legal system, in order to investigate whether and how the concept may affect the value of damages due to the creditor. In this sense, after conceptualizing the opportunity cost, an analysis is made of the function of civil liability, as well as the method of calculating damages. Once this framework is established, the aim is to contribute to the debate regarding the necessary requirements for two distinct indemnity claims related to opportunity cost, frequently observed in disputes, to be met: claims related to the opportunity cost of invested amounts and claims related to the opportunity cost of amounts due bot unpaid in whole or on time to the creditor.