BACKGROUND: Seriously ill patients often depend on their informal caregivers to help and support them through the disease course. This study investigated informal cancer caregivers' experiences of caregiving tasks and consequences and how caregiver status (primary vs. non-primary caregiver) and the caregiver's relationship to the patient (spouse/partner, etc.) are related to these experiences. METHODS: In a cross-sectional questionnaire study, randomly selected cancer patients with a range of diagnoses and disease stages were invited to pass on the 'Cancer Caregiving Tasks, Consequences and Needs Questionnaire' (CaTCoN) to 1-3 of their caregivers. RESULTS: A total of 590 caregivers related to 415 (55% of 752 eligible) cancer patients participated. Large proportions of caregivers experienced substantial caregiving workload, e.g., provision of psychological support (74%), as well as a range of negative consequences, most commonly stress (59%). Some caregivers experienced personal growth, but relatively large proportions did not. Caregiver status and the caregiver's relationship to the patient were associated with some caregiving aspects. Primary caregivers experienced the highest caregiving workload, and non-primary caregivers experienced most problems with getting time off from work. Spouses/partners and/or parents experienced the highest workload, most lack of time for social relations, most financial difficulties, and had the greatest need for seeing a psychologist. They furthermore experienced the highest degree of personal growth and had the smallest need for living a normal life while being a caregiver. Yet, regarding the majority of caregiving aspects, no associations with caregiver status or the caregiver's relationship to the patient were found. CONCLUSIONS: Overall, the findings confirm that cancer caregiving is burdensome. The primary and the closest caregivers seemed to take on most caregiving tasks, but, contrary to expectations, regarding the majority of caregiving consequences non-primary and more distant caregivers were affected to the same degree as the primary and closest caregivers. Initiatives and interventions to support not only the primary caregivers are therefore warranted.