Objectives The aim of this study was to assess the impact of adding assistants in nursing to acute care hospital ward nurse staffing on adverse patient outcomes using administrative health data. Design Logistic regression modelling was used with linked administrative health data to examine the association between seven adverse patient outcomes and use of assistants in nursing utilising a pre-test/post-test design. Outcomes included were in-hospital 30-day mortality, failure to rescue, urinary tract infection, pressure injury, pneumonia, sepsis and falls with injury. Setting Eleven acute care metropolitan hospitals in Western Australia. Sample Patients were retained in the dataset if they spent any time on a medical, surgical or rehabilitation ward during their admission and excluded if they only spent time on other ward types, as the outcomes used in this study are only validated for these patient populations. There were 256,302 patient records in the total sample with 125,762 in the pre-test period (2006–2007) and 130,540 in the post-test period (2009–2010). Results The results showed three significant increases in observed to expected adverse outcomes on the assistant in nursing wards (failure to rescue, urinary tract infection, falls with injury), with one significant decrease (mortality). On the non-assistant in nursing wards there was one significant decrease (pneumonia) in the observed to expected adverse outcomes and one significant increase (falls with injury). Post-test analysis showed that spending time on assistant in nursing wards was a significant predictor for urinary tract infection and pneumonia. For every 10% of extra time patients spent on assistant in nursing wards they had a 1% increase in the odds of developing a urinary tract infection and a 2% increase in the odds of developing pneumonia. Conclusion The results suggest that the introduction of assistants in nursing into ward staffing in an additive role should be done under a protocol which clearly defines their role, scope of practice, and working relationship with registered nurses, and the impact on patient care should be monitored.