Primary leiomyosarcoma of the colon mesentery is an extremely rare neoplasm, and only a small number of cases have been reported. We describe a case of leiomyosarcoma originating in the colonic mesentery, in a 68-year-old woman. Ultrasound showed a heterogeneous mass with varying vascularization in the left fossa. Central areas of the mass were hypoechoic, without detectable vascularization. Contrast enhanced computed tomography (CECT) of chest and abdomen showed a contrast enhanced tumour, with central non-enhanced areas. The tumour was radically resected and histopathology showed primary leiomyosarcoma. Two years after primary surgery, follow-up CECT revealed a local recurrence, which was re-resected. Subsequent follow-up CECT since have shown no sign of recurrence.