A case of subclavian vein thrombosis which was not detected clinically is described. Doppler ultrasound was used to ascertain the diagnosis. The technique of examination with the Doppler method for upper limb thrombi is described. It is expected that this technique will be as valuable for detection of thrombosis in the upper extremities as it has been shown to be in lower extremities. This is important because the incidence of deep vein thrombosis has increased with the frequent use of indwelling central venous catheters and because of increased awareness of the possibility and dangerous sequelae of the thrombosis, Early detection of thrombus by this non-invasive method, with removal of the catheters and prompt heparin therapy can preserve the integrity of the venous conduits and avert emboli. This is particularly relevant for anaesthetists in critical units.