A 49-year-old woman presented with a uterine tumour with solid and cystic parts. Extension of the tumour into the broad ligament was noted at the operation with wormlike plugs of tumour within the veins. The histological diagnosis of intravenous leiomyomatosis was evident. The disease turned out to be far advanced: a long continuous intraluminal mass extending from the iliac veins to the right ventricle made a second operation with thoracotomy and laparotomy mandatory. Postoperative follow-up showed so far, 1.5 years after operation, no evidence of recurrence. Intravenous leiomyomatosis is a very uncommon finding. Though it is histologically a benign smooth-muscle tumour, the biological behaviour with intravenous growth which may involve the right side of the heart, is quite aggressive. About 100 cases were reported in the literature; in no case could the diagnosis be made before surgery.