Patients with suspected deep vein thrombosis (DVT) are often managed on an outpatient basis by primary care physicians. International guidelines recommend anticoagulant treatment for patients with suspected DVT when diagnostic testing is delayed or when clinical probability is high. Our goal was to build a clinical prediction rule specifically for easy use in primary care to help decide about starting anticoagulant therapy while awaiting ultrasound examination. Between January and December 2006, 276 patients with clinically suspected DVT were included in this study by 189 general practitioners from Brittany, France. All patients underwent a standardized clinical assessment and were then referred for ultrasonography. The diagnosis of DVT was confirmed in 103 (37%) patients. The final clinical prediction rule comprises four risk factors for DVT (personal history of venous thromboembolism +1, immobilization in previous month +1, estrogen contraceptive +2, active malignancy +3), one clinical sign (swelling of the calf +1), and the presence of an alternative diagnosis more likely than that of DVT (-3). The proportion of confirmed DVT was 26% in patients classified as at low risk, with a score less than 2 points, and 63% in patients classified at high risk, that is, with a score of 2 points or more. This clinical prediction rule is based on simple history and clinical factors that are routinely collected by GPs from patients with suspected DVT. It could help to decide about the immediate prescription of anticoagulation pending ultrasound. This rule should be externally validated before its use in clinical practice can be recommended.