ABSTRACTObjective:To describe the implementation of Modified Seldinger Technology for percutaneous catheterization in critically ill newborns.Method:A quasi-experimental before- and-after study, carried out with neonatologist nurses in a Neonatal Intensive Care Unit.Results:Seven nurses participated in the research. Catheter pre-insertion, insertion and maintenance were assessed using the conventional and modified Seldinger technique. Reliability was satisfactory in pre-test, 5.40 (Md = 6.00), and post-test, 5.94 (Md = 7.00), and perfect in the items about device insertion and maintenance. There was low assertiveness in the items on indication, microintroduction procedure via ultrasound, limb repositioning and disinfection of connections/connectors.Conclusion:Despite the Modified Seldinger Technique expanding some stages of execution over the traditional method of percutaneous catheterization, nurses were more assertive after theoretical-practical training. The technology was implemented and is in the process of being implemented in the health service.