The search for an unknown primary tumour is often time-consuming, costly and unrewarding. Positron emission tomography might be an effective method for screening the body for malignant deposits. We present the case of a woman with a symptomatic liver tumour of unknown origin. Several investigations did not reveal a primary tumour, but PET scanning showed a hot spot in the pelvis, suggesting either a primary tumour or a metastatic deposit. During operation, a primary Fallopian tube carcinoma was detected. Histopathological examination of the resected liver tumour revealed a metastasis of the Fallopian tube carcinoma. This case report demonstrates that PET scanning can be useful in the diagnostic process in patients with unknown primary tumour, and that a symptomatic liver tumour can be the first sign of Fallopian tube carcinoma.