Aim:  To  evaluate  the  knowledge  of  mothers  regarding  adverse consequences from the pentavalent vaccination before and after they participated in an educative  intervention.  Method:  This is  a  quasi-experimental  study,  with  a  before-and-after analysis, of a single group of 100 mothers of breastfeeding babies, from September to October 2014, in an outpatient clinic of a public maternity hospital in the municipality of  Imperatriz,  Brazil.  Results:  Following  the  intervention, a  significant  increase  in understanding  was  observed  among  the  study  participants  in  all  questions  (p<0.05) regarding adverse events after the pentavalent vaccine. After the intervention, there was a significant increase of right answers to the questions: who to seek advice from after an adverse  event  due  to  the  vaccine  (p=0.004)  and  which  adverse  events  from  the pentavalent vaccine are known to mothers (p=0.04). Conclusion: The results indicate a positive change in the  level  of  understanding  of  mothers regarding the  adverse  events after the  pentavalent vaccination.  The  study  provides  a  basis  for the  adoption  of long-term intervention strategies for nurses.