Critical ischaemia in the chronically ischaemic limb has proved difficult to define both in clinical and objective terms. This has meant that assessment of patients has in the past relied heavily on the angiographic appearances of the vessels which is also an unreliable technique. Attempts have been made to define critical ischaemia objectively in terms of the ankle systolic pressure but recent studies have shown that although the definition produced in 1982 was highly specific, the sensitivity was low with many patients with the clinical features of critical ischaemia having ankle pressures which were higher than the recommended values. Recently the transcutaneous measurement of oxygen tension has been used to assess patients with peripheral vascular disease and has proved of use in assessing distal tissue perfusion in chronically ischemic limbs. This study compares ankle systolic pressure measurements with the transcutaneous oxygen tension results from the ischaemic foot with the patient sitting and found that the latter gave a more accurate guide to the severity of ischaemia and a more accurate prediction of the short-term results in individual cases.