BACKGROUND Thrombosis of the central veins is one of the most frequent complications of implanted venous access devices. Among the first cases occurring in our patients, most were associated with left-sided placement of the ports, with catheter tips lying against the external wall in the upper half of the superior vena cava. Some chest radiographs showed lateromediastinal opacities centered on the catheter tip, suggesting a vessel injury. This position allows a narrow contact between the catheter tip and the vessel wall, thus endothelial injuries might result from mechanical and chemical attack. METHODS To assess the role of catheter position, we reviewed the routine chest radiographs of 379 patients who received chemotherapy through venous access devices and were followed up at our department between December 1985 and December 1990. Four groups (upper left, upper right, lower left, and lower right) were defined according to the level of the catheter tip (innominate veins or upper half of the vena cava versus lower half of the vena cava or auricula) and to the side of port implantation. RESULTS Ten patients developed symptomatic venous thrombosis (superior vena cava in 9 patient, left subclavian vein in 1 patient). A strong correlation existed between catheter position and incidence of thrombosis: upper left, 8/28 (28.6%); upper right, 1/33 (3%); lower right, 1/68 (1.5%); and lower left, 0/250. Since 1988, we have insisted on replacement of malpositioned catheters, and we have observed fewer thromboses (2/191 versus 8/188). CONCLUSIONS The current study suggests that patients with left-sided ports and catheter tips lying in the upper part of the vena cava are at high risk for severe thrombotic complications.