Background: Family caregivers play a key role in the lives of patients with multiple myeloma. However, very little is known about the impact that the disease (its diagnosis, course and prognosis) has on the main family caregiver.; Aim: To achieve a deeper understanding of the lived experience of individuals who are the primary caregiver of a relative with multiple myeloma and to shed light on their needs.; Design: Interpretative phenomenological study.; Setting and Participants: A total of 12 individuals who were the main family caregivers of a relative with multiple myeloma who was under outpatient follow-up at a cancer unit in Barcelona were recruited via purposive sampling until data saturation was reached. In semi-structured in-depth interviews, participants described their experiences of caring for their relative with multiple myeloma. Interviews were recorded, transcribed and analysed using ATLAS.ti v7.2. The seven steps proposed by Colaizzi were used for data analysis, and the relationships among emerging themes were examined.; Findings: Four main themes emerged: (a) a new life, adapting to the disease, (b) commitment to the patient, (c) the emotional sphere and (d) experiences in relation to the care and support received. The analysis also revealed a core overarching theme: uncertainty.; Conclusion: Primary family caregivers experienced intense uncertainty, and they described a strong need to air their feelings. Specific practical initiatives, targeting both health-related and logistical aspects of care, need to be developed in order to support family caregivers of myeloma patients.