this study aims to present the cross-cultural adaptation process of the modified dyspnea index to the brazilian culture and to investigate its content validity and reliability. this process included the steps of translation, back translation and review by two experts to assess semantic, conceptual, idiomatic, cultural and metabolic equivalence. the index of content validity was used to evaluate the extent of inter-observer agreement. a guide to implement the modified dyspnea index was developed and validated. two different professionals assessed the reliability of the brazilian version of the modified dyspnea index, according to the inter-observer equivalence criterion, with 31 patients, indicating a kappa coefficient=0.960 (p<0.001). in conclusion, the brazilian version of mdi presented evidence of interobserver equivalence when applied by different health professionals in the population of cardiac patients.