Patient-related complexity of nursing care in acute care hospitalsan updated concept Rational: The interest in complexity of nursing care has grown in science and practice in recent years because of changed patient profiles and higher average levels of patient acuity in acute care hospitals. Aim: The aim of this study was to redefine the concept of patient-related 'complexity of nursing care' in acute care hospitals. Design: The hybrid model for concept development was chosen. Methods: In a first theoretical phase, we performed a narrative literature review regarding defining elements of patient-related 'complexity of nursing care' and developed a working definition. In the fieldwork phase, we investigated collective case studies representing 12 nursing care situations. The theoretical phase was updated before all findings were synthesised, and in the final analytical phase, the preliminary working definition was refined. Ethics: An ethical committee judged the study as unproblematic (waiver no. 82/14 from 5 December 2014). Results: The concept was redefined confirming previous understandings of patient-related 'complexity of nursing care' as a dynamic, relational concept characterised by the instability, variability and uncertainty that exist. The extent of complexity was determined by multifaceted patient problems and resources as well as extensive knowledge, experience, attention and caring skills of registered nurses interacting with each other. Limitations: The study did not include organisation-related complexity of nursing care. Conclusions: The redefinition of the concept may support a common understanding of patient-related demands on nursing care among practitioners, managers and politicians which is important regarding patient safety and health nursing staff.