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Patients with carcinoma of unknown primary origin (CUP) pose a difficult diagnostic challenge, but a missed diagnosis of the primary tumor site is, unfortunately, a frequent occurrence. Musser and colleagues describe a patient with a large para-aortic mass that was initially diagnosed as CUP, but was subsequently identified as metastatic seminoma that showed remarkable spontaneous regression. Background. A 63-year-old man presented to an emergency department with abdominal and back pain. A 5 × 3 × 2 cm para-aortic mass was found adjacent to the left kidney, but the location of the primary tumor could not be identified. He was referred to a tertiary center with a diagnosis of retroperitoneal carcinoma of unknown primary origin. Atrophy of the left testicle was noted. Review of the biopsy material obtained at the first presentation showed histology characteristic of seminoma, and imaging revealed the presence of small intratesticular lesions. Repeat CT showed complete resolution of the metastatic para-aortic mass. The patient underwent left orchiectomy and adjuvant radiation therapy, and showed no signs of disease 6 months later. Investigations. CT of the chest, abdomen and pelvis, measurement of serum tumor markers, upper gastrointestinal endoscopy, colonoscopy, PET–CT and percutaneous needle biopsy of the para-aortic mass at the first presentation; physical examination, review of biopsy specimen with immunohistochemical staining for tumor markers, testicular ultrasonography and repeat CT of the chest, abdomen and pelvis at the second presentation. Diagnosis. Metastatic seminoma with spontaneous regression of the metastatic lesion. Management. Radical orchiectomy and adjuvant radiation therapy.