Objective: To explore the applicability of a patient complaint taxonomy to data on serious complaint cases. Design: Qualitative descriptive study. Setting: Complaints made to the New South Wales (NSW) Health Care Complaints Commission, Australia between 2005 and 2010. Participants: All 138 cases of serious complaints by patients about public hospitals and other health facilities investigated in the 5-year period. Main Outcome Measure: A thematic analysis of the complaints was conducted to identify particular complaint issues and the Reader et al. (Patient complaints in healthcare systems: a systematic review and coding taxonomy. BMJ Qual Saf 2014;23:678-89.) patient complaint taxonomy was then used to classify these issues into categories and sub-categories. Results: The 138 investigated cases revealed 223 complaint issues. Complaint issues were distributed into the three domains of the patient complaint taxonomy: clinical, management and relationships. Complaint issue most commonly related to delayed diagnosis, misdiagnosis, medication errors, inadequate examinations, inadequate/nil treatment and quality of care including nursing care. Conclusions: The types of complaints from patients about their healthcare investigated by the NSW Commission were similar to those received by other patient complaint entities in Australia and worldwide. The application of a standard taxonomy to large numbers of complaints cases from different sources would enable the creation of aggregated data. Such data would have better statistical capacity to identify common safety and quality healthcare problems and so point to important areas for improvement. Some conceptual challenges in devising and using a taxonomy must be addressed, such as inherent problems in ensuring coding consistency, and giving greater weight to patient concerns about their treatment.