Teacher's job is highly demanding in what concerns the cognitive performance. Once in the classroom the exposure to noise might be significative and a strong risk factor for these professionals, as the performance in their highly cognitive tasks is so required. According to this premise, the present study aims to find out the relationship between noise exposure in the classroom and the teacher's cognitive performance. Theoretically, this study deals with the concept of noise and its effects bearing in mind that the individual sensitiveness to noise is a subjective parameter. Consequently, its measurement implies the use of an previously validated scale known as the Weinstein's Noise Scale. Empirically this study starts with the selection of a school where 16 teachers, teaching different school subjects, were chosen and divided in two groups: Practice (P) and Theoretical-practice (TP). In these teachers' classrooms, the daily exposure levels were measured during 4 weeks. To evaluate how much the noise interferes in their cognitive performance, they were tested during 4 weeks in 2 periods of the day. The analysis of the results shows that the P group had a better performance than the TP. However, both groups showed a decrease in their performance after being exposed to classroom noise. Concerning to the individual sensitiveness to the noise, it showed that the "non-sensitive" (or less sensitive) subjects had better performance than the noise sensitive subjects' group and, again, both groups showed a decrease in their cognitive performance under the same circumstances. The results showed that there is a relationship between noise exposure and teachers' cognitive performance although these results may not be consistent when analysing all other variables.