In primary care, the physician has to decide which patients with a suspicion of deep vein thrombosis (DVT) have to be referred for further diagnostic work-up. Accurate referral is of utmost importance because unrecognized and therefore untreated DVT may cause pulmonary embolism. The classic clinical findings are not sufficiently accurate for the diagnosis of DVT. The majority of the referred patients, 70 to 80%, do not have DVT and this puts a burden on both patients and health care budgets. Diagnosis in primary care is different from that in secondary care caused by the referral mechanism or spectrum difference. Diagnostic tests derived in secondary care, therefore, cannot simply be generalized to primary-care patients. The well-known diagnostic rule for DVT, the Wells rule, does not adequately rule out DVT in primary-care patients. A proper diagnostic rule for use in primary care is lacking; therefore, we investigated the data of 1,295 patients in primary care suspected of having DVT. We developed and validated a simple diagnostic decision rule to exclude the presence of DVT safely in primary care. Independent diagnostic indicators of the presence of DVT were male gender, oral contraceptive use, presence of malignancy, recent surgery, absence of leg trauma, vein distension, calf circumference difference, and D-dimer test result. Application of this rule could reduce the number of referrals by at least 23%, whereas only 0.7% of the patients with a DVT would not be referred. A diagnosis strategy is given, together with a practical flow diagram.