Objectives To investigate the relationship between behavioural problems in patients with dementia and changes in the marital relationship. Methods Fifty-three spouse caregivers of patients with dementia participated in the study. Questionnaires and interviews were used to examine caregiver perception of changes in the quality of their relationship. Behavioural disturbances in the patient were measured with the NeuroPsychiatric Inventory (NPI). Results Caregivers experienced a deterioration of their relationship, yet at the same time most felt closer to their spouse now than in the past. Regression analysis revealed that patient behavioural problems were, independent of patient cognitive status or functional impairment, associated with deterioration in the quality of the relationship between patient and caregiver. Patient apathy rather than depressive mood was associated with this deterioration. Apathy diminished the amount and reciprocity of interactions between partners. Conclusions These results show that passive behaviour rather than excessive behaviour has most impact on the deterioration of the marital relationship. Intervention programmes should target relationship problems when problem behaviour, especially apathy, is present in patients with dementia.