This is a study of a quanti-qualitative nature carried out in a public maternity located in the city of João Pessoa-PB, having as an objective to investigate the knowledge that the mothers have concerning the PKU Test. The sample was made up of 110 women who gave birth in this maternity and/or came for pre-natal service there. The results showed that 97% of the mothers had heard about the test and 99% thought that the PKU Test was important. Despite these results, the data reveals that the mothers are not aware of its purpose and the importance of the PKU Test.