Data regarding the prevalence of dementia and the prevalence of cancer suggest that a relatively large number of older people living with dementia may also have unsuspected comorbid cancer. Research into dementia and comorbid cancer is limited. The existing literature shows that people living with dementia have much lower cancer survival rates than people who do not have dementia, perhaps due to advanced age and cancer being diagnosed at a late stage or not at all. In people with dementia, investigating and managing cancer is complicated by cognitive impairment, atypical presentation, communication difficulties and potential behavioural issues – all of which may deter healthcare professionals from conducting patient assessments and screening tests. In this article, the authors use a case study to illustrate the challenges that can arise when a person living with dementia develops cancer, particularly in relation to diagnosis, pain assessment, decision-making and carer involvement.