(1) Heparin prophylaxis for medical inpatients who are confined to bed is controversial, because there are no reliable comparative data. Prophylaxis only seems justified for some patients at high risk of pulmonary embolism who have no risk factors for bleeding. (2) The licence of dalteparin, a low-molecular-weight heparin (LMWH), has been extended in France to cover prophylaxis of deep venous thrombosis in patients confined to bed for heart failure, acute respiratory failure, acute infections or acute rheumatological conditions who have at least one other risk factor for venous thromboembolism. (3) Evaluation data in this setting include a placebo-controlled trial but no trials versus unfractionated heparin or enoxaparin (another LMWH already available for this use). (4) The PREVENT trial included 3681 patients matching the characteristics described in the licence. They were randomised to receive (double-blind) daily subcutaneous injections of dalteparin (5000 IU) or placebo for 14 days. There was no difference between the groups in the following outcomes: death, pulmonary embolism and venous thrombosis (incidence below 1% in the placebo group). The results based on an endpoint combining clinical outcomes and phlebographic abnormalities favoured dalteparin. (5) Few data are available on the adverse events occurring in this trial. In other clinical situations, dalteparin has the same adverse effects as other LMWH (thrombocytopenia, hyperkalemia, etc.). (6) In practice, for medical inpatients who are confined to bed, where the thromboembolic risk is low, dalteparin offers no tangible advantages over unfractionated heparin or enoxaparin. The optimal dose for preventing symptomatic thromboembolism and minimising the bleeding risk is unknown.