Objectives: To study the determinants of health-related quality of life (HRQoL) in Irish patients with diabetes using the Centres for Disease Controls' (CDC's) 'Unhealthy Days' summary measure and to assesses the agreement between this generic HRQoL measure and the disease-specific Audit of Diabetes Dependant Quality of Life (ADDQoL) measure.Research Design and Methods: Data were analysed from the Diabetes Quality of Life Study, a cross-sectional study of 1,456 people with diabetes in Ireland (71% response rate). Unhealthy days were assessed using the CDC's 'Unhealthy days' summary measure. Quality of life (QoL) was also assessed using the ADDQoL measure. Analyses were conducted primarily using logistic regression. The agreement between the two QoL instruments was measured using the kappa co-efficient.Results: Participants reported a median of 2 unhealthy days per month. In multivariate analyses, female gender (P = 0.001), insulin use (P = 0.030), diabetes complications (P =<0.001) were significantly associated with more unhealthy days. Older patients had fewer unhealthy days per month (P = 0.003). Agreement between the two measures of QoL (unhealthy days measure and ADDQoL) was poor, Kappa = 0.234Conclusions: The findings highlight the determinants of HRQoL in patients with diabetes using a generic HRQoL summary measure. The 'Unhealthy Days' and the ADDQoL have poor agreement, therefore the 'Unhealthy Days' summary measure may be assessing a different construct. Nonetheless, this study demonstrates that the generic 'Unhealthy Days' summary measure can be used to detect determinants of HRQoL in patients with diabetes.