Eight cases are reported of extensive retroperitoneal haemorrhage in patients receiving anticoagulant therapy who subsequently died and underwent postmortem examination. No patient had an obvious source of bleeding, and in 5 cases no other cause for death was apparent. Three patients had had vascular surgical procedures: the other 5 were given anticoagulants for suspected venous thrombo-embolism. Six cases presented as unexpected circulatory collapse with rapid demise. Four patients had in addition gastro-intestinal bleeding of varying degree, with no obvious source. This acute massive type of retroperitoneal bleeding contrasts with the commoner presentations of pain, swelling, bruising and compression neuropathy: a high index of suspicion and urgent treatment are required if mortality is to be reduced.