Arterial catheter tip position and arterial flow was monitored by intra-arterial infusion of 99Tcm macroaggregated albumin in patients treated by intra-arterial chemotherapy. Variations in arterial flow pattern as monitored by macroaggregated albumin perfusion studies were encountered in 16 patients without apparent change in the catheter position as monitored by arteriographic and/or radiographic examinations. Possible mechanisms causing changes in arterial flow pattern include: (a) development of collateral circulation, (b) arterial spasm, (c) progressive arteritis causing thrombi with or without associated spasm, and (d) laminar flow. It is important to recognise that changes in perfusion patterns are not always accompanied by change in the position of the arterial catheter.