An 81 year old man presented with an asymptomatic swelling of his left upper limb. The patient's past medical history included angina, hypertension, and a stroke (five years previously) which left him with mild dysphasia, but no other deficit. There was no past medical or family history of thrombosis. There was a two day history of swelling. The arm was not painful, hot or red, and there was no history of trauma. Apart from pain in the left side of his neck, which had been present for three weeks, the patient was well. System