BACKGROUND: Research into burden among spouse and adult-child caregivers of patients with Alzheimer's disease has generated contradictory results as regards the group which suffers the greatest burden and the factors underlying any differences. OBJECTIVES: The aim of the present study was to identify and compare the factors associated with caregiver burden among spouse and adult-child caregivers. DESIGN: Cross-sectional analytic study. SETTINGS: All clinical subjects had been referred on an out-patient basis to the Memory and Dementia Assessment Unit of the Santa Caterina Hospital in Girona (Spain). PARTICIPANTS: Data were collected from 251 patients and their caregivers, 112 with spouse and 139 with adult-child caregivers. METHODS: The association between caregiver burden and the socio-demographic and clinical variables of both patients and caregivers was analysed, the results being compared for spouse vs. adult-child caregivers. Burden was analysed using a multivariate linear regression including all the variables for the two groups of caregivers. RESULTS: The results show greater burden among adult-child caregivers (p<.05), who experience more feelings of guilt (p<.001). In both groups the behavioural and psychological symptoms of patients were correlated with burden (p<.001). Living with the patient has a notable influence on burden among adult children (p<.001). Husbands, wives, daughters and sons, in this order, showed increasing levels of burden (p<.05) and progressively worse mental health (p<.01). However, the correlations between burden and mental health were strongest in daughters (p<.001). CONCLUSION: The differences in burden between spouse and adult-child caregivers were not associated with age, physical health or clinical factors of the patients. Overall burden was greater among adult-child caregivers, especially those who lived with the patient and who had other family duties. Feelings of guilt were associated with not living with the patient, and there was a strong correlation between burden and mental health. These results support the hypothesis that spouses regard caregiving as part of their marital duties, whereas for adult children such tasks imply an important change in their lifestyle.