A Border collie was presented at the age of 9 weeks with several lesions of the right forelimb, including a reddish-blue haemangiomatous macula in the medio-dorsal part of the elbow, multiple, scattered small cavernous haemangioma-like lesions at the plantar part of the foot and a general hypertrophy of the limb. X-rays of the limb showed osteolysis. On skin biopsy, telangiectatic veins were observed. The rest of the body did not show any skin lesions or hypertrophy. The dog was otherwise healthy. Due to the extension of the lesions and worsening of the limb swelling, it was decided to amputate the affected limb. The dog remained healthy for 2 weeks, but then passed through episodes of anaemia, and finally died suddenly with signs of shock. Dissection of the limb after amputation revealed hypoplasia and aplasia of the deep venous system in the lower part of the leg. No arterio-venous shunts were noticed. In man, this syndrome, characterised by an insufficiently developed deep venous system associated with local overgrowth of the limb and cutaneous telangiectasia, is known as Klippel-Trenaunay syndrome.