The relationship between thought/language and sign language is discussed in a study about the resolution of mathematical problems, involving deaf subjects aged between 18 and 30, public school students from the initial grades of the Educational Program for Young Adults and Adults in the Distrito Federal (federal capital of Brazil). The study was developed in three phases: evaluation of the subjects' mathematical competencies, regarding the logic of the numerical system and its notation; research about the expressions that best translated "greater than n" and "fewer than n" in LIBRAS (Brazilian Sign Language) in situations where sets were compared; investigation of mathematical problem solving in two different situations: the subject's resolution of the problems with and without intervention. The results suggest that deaf people's difficulties in relation to mathematical problems are connected to the process of schooling due to its emphasis on the acquisition of solving procedures to the detriment of conceptual acquisition. Yet, the misusage of LIBRAS as a tool that favors the organization of semiotic meanings and knowledge acquisition jeopardizing the construction of deaf people's knowledge in this area.