To establish the best approach to develop a quality of life (QoL) questionnaire for cancer-patient caregivers, this study attempts to identify primary domains of QoL in terms of their impact on a purposive sample of caregivers. Seventy-seven informal adult caregivers of cancer patients (breast cancer, paediatric haematological malignancies or melanoma) with different relationships with the patients (parents, children, spouses, siblings, and friends) were recruited at three specialised French centres and extensively interviewed. Caregivers' lives were altered in several domains: psychological well-being, leisure and everyday activities, relationships with institutional caregivers, occupation and finances, relationships with family and friends, physical well-being, and relationship with the patient. The relative importance of these domains varied mainly in association with the caregiver-patient relationship. Multiple correspondence analysis identified two isolated clusters: children, and, most significantly, friends and siblings. The latter groups emphasised the repercussions on their psychological well-being and their relationship with the patient, but were less willing to discuss the impact on their relationship with caregivers and on occupation, finances, leisure, and everyday activities. This study focuses on the caregiver's perspective and advocates the development of a short QoL core questionnaire. Additional modules should be cancer-specific or dedicated to specifics of the caregiver-patient relationship.