Aims: To explore key factors related to nursing unit instability, complexity and patient and system outcomes. Background: The relationship between nurse staffing and quality of patient outcomes is well known. The nursing unit is an important but different aspect that links to complexity and to system and patient outcomes. The relationship between the instability, complexity and outcomes needs further exploration. Design: Descriptive. Methods: Data were collected via a nurse survey, unit profile and review of patient records on 62 nursing units (wards) across three states of Australia between 2008-2010. Two units with contrasting levels of patient and nurse instability and negative system and patient outcomes, were profiled in detail from the larger sample. Results: Ward A presented with greater patient stability (low occupancy, high planned admissions, few ICU transfers, fewer changes to patient acuity/work re-sequencing) and greater nurse instability (nurses changing units, fewer full-time staff, more temporary/casual staff) impacting system outcomes negatively (high staff turnover). In contrast, Ward B had greater patient instability, however, more nurse stability (greater experienced and permanent staff, fewer casuals), resulting in high rates for falls, medication errors and other adverse patient outcomes with lower rates for system outcomes (lower intention to leave). Conclusion: Instability in patient and nurse factors can contribute to ward complexity with potentially negative patient outcomes. The findings highlight the variation of many aspects of the system where nurses work and the importance of nursing unit managers and senior nurse executives in managing ward complexity.