Background Patients with cancer of unknown primary (CUP) have metastatic malignant disease without an identifiable primary site; it is the fourth most common cause of cancer death.

Objectives To explore patients’ informal and professional carers’ experiences of CUP to inform development of evidence-based, patient-centred care.

Methods Qualitative study involving development of multiple exploratory case studies, each comprising a patient and nominated informal and professional carers, with contextual data extracted from medical records.

Results 17 CUP patients, 14 informal and 13 professional carers participated in the study. Two inter-related themes distinct to CUP emerged: uncertainty and continuity of care. In the absence of a primary diagnosis, patients and informal carers experienced uncertainty regarding prognosis, possible recurrence and the primary's hereditary potential. Professional carers experienced difficulty communicating uncertainty to patients, ambiguity in deciding optimal treatment plans in the absence of trial data and a test or treat dilemma: when to discontinue seeking the primary and start treatment. Common problems with care continuity were amplified for CUP patients relating to coordination, accountability and timeliness of care. The remit of multidisciplinary teams (MDTs) often excluded CUP, leading to “MDT tennis” where patients were “bounced” between MDTs.

Conclusions The experience of those with CUP is distinctive and it can serve to amplify some of the issues encountered by people with cancer. The clinical uncertainties related to CUP compound existing shortcomings in continuity of care, increasing the likelihood of a disrupted patient journey. However, while little can be done to overcome uncertainty, more could be done to address issues regarding continuity of care.