The frequency of venous thrombosis during central vein catheterization was prospectively studied in a group of 51 patients (53 catheterizations) during a 10-month period. 12 catheterizations (23%) resulted in thrombosis as revealed by phlebography. Of these, 11 were due to catheters whose site of entry was the upper limb. No other favoring factors (osmolality of the infused fluids, length or catheterization etc.) were detected. Ideally, to avoid thrombosis, central venous catheters should be inserted via the jugular or the subclavian vein, while bearing in mind that these other routes are not devoid of complications either.