The impact of comorbidity on patient outcomes following an intervention has been largely ignored. No studies have been reported in the UK or Japan. The aim of this thesis was to assess the impact of comorbidity on the outcome of a common major surgical operation - total hip replacement. Comorbidity was measured using the Index of Co-Existent Disease developed in the USA, which reliability was assessed. Two retrospective cohorts, one in Japan and one in the UK were studied. Data were collected from patients' case notes extraction and by postal questionnaire to patients one year after surgery. After THR, patient's health status was improved in both countries and satisfaction for care was high. Significant differences in in-hospital complications were observed between Japan and the UK in terms of complication rate. type and severity, and their association with independent variables. Comorbidity was significantly associated with serious complications and with change in health status in the UK and with minor complications in Japan. A logistic regression model using the ICED and independent confounding factors suggested a significant relationship between comorbidity and complications. However, the model did not fit the data well. A multiple regression model for change in health status showed that much of the variance was explained by the preoperative health status but not by comorbidity. The low number of serious complications in Japan and the high complication rate in patients in the lowest comorbidity severity level in the UK made the predictive power weak. Finally, through the experience of this study, some recommendations for clinical practice and further research are discussed.