We investigated the risk factors for venous thrombosis in cancer patients with implantable ports undergoing chemotherapy. One hundred and seventy one ports were placed in a central ("chest ports") and 84 in a peripheral vein ("arm ports"), 181 received prophylactic nadroparin and 10 coumarin. Clinically overt thrombosis was confirmed by ultrasound or angiography. Catheter-related thrombosis incidence without anticoagulants was 28% in arm and 33% in chest ports, but with anticoagulants this was 32% in arm and only 1% in chest ports (odds ratio (OR) 34.8 95% confidence interval (CI) 7.3-165). Left-sided placement compared with right-sided and catheter tip position in the superior vena cava compared with right atrium were associated with a 3.5 respectively 2.6-fold increased risk. Thrombosis was associated with elevated homocysteine levels (OR=3.8, 95% CI 1.3-11.3), but not with factor V Leiden or prothrombin 20210A gene mutations, or high concentration of factor VIII, IX or XI. Prophylaxis with anticoagulants is recommended for chest, but not for arm ports. Determination of plasma homocysteine levels may identify patients at an increased risk for thrombosis.