Background and purpose Endotracheal tube suction performed in children can affect clinical stability. Previous research has identified clinical indicators used to perform endotracheal suction. These were used to develop the Endotracheal Suction Assessment Tool© (ESAT©). This study sought to evaluate the degree to which the ESAT© items as a whole constitute an operational definition of the construct used to determine whether a paediatric intensive care nurse should perform the endotracheal tube suction procedure. Methods Lynn's process for calculation of content validity and scale content validity index using a team of expert reviewers was adopted. Experts were drawn from paediatric intensive care units in Australia (n = 6), United Kingdom (n = 1), Switzerland (n = 1) and Canada (n = 1). These experts established the content validity index of the Endotracheal Suction Assessment Tool© using a minimum preset a-priori criterion agreement of 0.78 and a scale content validity index of 0.8. Scale content validity index was used to enhance the interpretability of the content validity data. Results All 15 items achieved the preset a-priori agreement for apparent internal consistency. Minor adjustments were required to improve the clarity of four items. The content validity index ranged from 0.8 to 1.0 and scale content validity index ranged from 0.9 to 1.0 for all items. Conclusion Item and scale content validity indexes of the tool were established. Further psychometric testing for construct validity and stability over time is required to establish clinical utility of the tool and practice of novice paediatric intensive care nurses and other PIC health professionals.