Limitations of magnetic resonance imaging and ultrasound-directed (duplex) scanning in the diagnosis of subclavian vein thrombosis. To investigate the potential role of magnetic resonance imaging and duplex scanning in the diagnosis of catheter-induced subclavian vein thrombosis, we correlated the results of 43 arm phlebograms with duplex scans; 28 of these phlebograms were also correlated with magnetic resonance imaging scans of the thoracic veins. Eighteen of the 43 phlebograms were normal, and all had normal magnetic resonance imaging and duplex studies. Eleven subclavian veins were totally occluded on phlebography; all had duplex scans, and five were also scanned with magnetic resonance imaging. Duplex scans detected 6 of 11 occlusions, whereas magnetic resonance imaging detected 4 of the 5 occlusions scanned. The five occlusions that were not detected by either magnetic resonance imaging or duplex scans were short segmental occlusions of the medial one third of the left subclavian vein. Of 14 nonocclusive thrombi seen on phlebography, duplex scans correctly identified 8. Magnetic resonance imaging was done on eight nonocclusive thrombi but identified only two. All abnormal findings on duplex scanning and magnetic resonance imaging were confirmed by phlebography. Short occlusions of the proximal portion of the left subclavian vein were often undetected by duplex scanning but occasionally seen with magnetic resonance imaging. Neither modality was sensitive to the presence of nonocclusive mural thrombi. Magnetic resonance imaging is highly reliable in ruling out the presence of a thrombotic process in the subclavian vein, but it may on occasion fail to detect the presence of subclavian thrombi. For this reason, in cases with suspected subclavian vein thrombosis magnetic resonance imaging cannot be used as the only diagnostic modality. 