OBJECTIVE: To assess the power of operative flow measurements in the prediction of outcome after infrainguinal bypass operations for critical ischaemia DESIGN: Open study SETTING: University department of vascular surgery SUBJECTS: 197 patients operated on for critical ischaemia between 1 January 1982 and 31 December 1989 under the care of one surgeon. INTERVENTIONS: Standard operative angiography and measurement of flow of distal runoff MAIN OUTCOME MEASURES: Patency and limb salvage RESULTS: All patients were followed up prospectively until 31 March 1991 or death. The estimated 5 year rates of primary patency and limb salvage were 0.75 and 0.86, respectively. Multivariate analysis showed that flow measurement was the most powerful predictor of graft patency (p = 0.0003) and limb salvage (p = 0.004) of all factors studied, including angiography. Irrespective of the site of distal anastomosis no polytetrafluoroethylene (PTFE) graft remained patent if the flow was less than 50 m/minute, whereas vein grafts remained patent if the flow was as little as 10 ml/minute. CONCLUSIONS: Flow is easy to measure and is a better predictor of outcome than angiography. Decisions about grafting should rely on flow measurements if there is any doubt about patency or limb salvage, particularly if a PTFE graft is going to be used.