Leiomyosarcomas (LMS) are rare malignant tumours of smooth muscle origin predominately affecting females in their sixth decade. Only 2% of LMS arise from blood vessels and most are in the inferior vena cava. We present the first reported case of LMS of the portal vein in a male patient. Multidetector CT showed a large mass in the main portal vein, which was initially misinterpreted as a pancreatic cancer. Careful examination of the multidetector CT images showed radiological features of an intraluminal mass, and a preoperative diagnosis of primary LMS of the main portal vein was made. The patient underwent curative surgery and made an uneventful recovery. Awareness of this entity and recognition of the salient CT features may facilitate radiologists in making the correct preoperative diagnosis.