BACKGROUND: The aim was to investigate the co-morbidity profile of people with dementia and examine the associations between severity of co-morbidity, health-related quality of life (HRQoL) and quality of life (QoL). METHODS: The improving the experience of Dementia and Enhancing Active Life (IDEAL) cohort consisted of 1,547 people diagnosed with dementia who provided information on the number and type of co-morbid conditions. Participants also provided ratings of their health-related and dementia-specific QoL. RESULTS: The majority of the sample were living with more than one chronic condition. Hypertension was commonly reported and frequently combined with connective tissue disease, diabetes and depression. The number of co-morbid conditions was associated with low QoL scores, and those with severe co-morbidity (≥5 conditions) showed the greatest impact on their well-being. CONCLUSIONS: Co-morbidity is an important risk factor for poor QoL and health status in people with dementia. Greater recognition of the nature and impact of co-morbidity is needed to inform support and interventions for people with dementia and a multidisciplinary approach to care provision is recommended.