A 63 year-old woman presented with a painful subcutaneous tumour on her right thigh. After ultrasound, suspicion of metastasis from malignant melanoma was raised. The tumour was subsequently excised. Histology proved the tumour to be a leiomyosarcoma (LMS) of the great saphenous vein. The patient was referred to a sarcoma center for further treatment. LMS of the great saphenous vein is a very rare tumour. From 1919 to 2004, 25 cases are reported in the literature. Diagnosis is aided by magnetic resonance imaging and Doppler ultrasound. Treatment is wide excision and sometimes radiotherapy. This kind of tumour can metastazise to the lungs. The survival prognosis is poor.