A case is presented of a 39-year-old male with Down's Syndrome, who also had type 1 diabetes of 22 years duration. He presented with diabetic ketoacidosis (DKA)—arterial blood pH 7.17, plasma bicarbonate 13.6mmol/l, plasma glucose 26.4mmol/l and urine heavily positive for ketones. He recovered with standard intravenous fluid and insulin treatment, but on the third day of admission developed a swollen left arm (which had not been used for intravenous cannulation). Doppler ultrasound confirmed a left axillary vein thrombosis. This slowly resolved with anticoagulation. Review of the available literature revealed that though arterial thrombosis is a common complication of DKA, venous thromboembolism is surprisingly rare, and there appear to be no previous specific reports of axillary vein thrombosis complicating DKA.