This article reports on a study on nurses' perception of power regarding their clinical role before and after implementation of a nursing diagnosis classification. Sixty clinical nurses (average age=37.2+/-7.0 years) from a Brazilian teaching hospital answered the Power as Knowing Participation in Change Tool (PKPCT) before and after the implementation of a diagnosis classification. PKPCT has four domains and provides total and partial scores. Reliability coefficients ranged from 0.88 to 0.98. Total scores were not statistically different between assessments (p=0.21), although scores in the 'Involvement in Creating Change' domain were higher in the second assessment (p=0.04). Further studies providing sound evidence regarding the impact of nursing classification systems on nurses' power perception are needed to guide decisions on teaching and clinical practice.