Context:The question of what is the best loading dosage of warfarin when starting anticoagulant treatment has been under discussion for ten years. We were unable to find any comparative studies of these characteristics conducted here in Brazil. Objective: To compare the safety and efficacy of two initial warfarin dosage regimens for anticoagulant treatment. Methods: One-hundred and ten consecutive patients of both sexes, with indications for anticoagulation because of venous or arterial thromboembolism, were analyzed prospectively. During the first 3 days of treatment, these patients were given adequate heparin to keep aPTT (activated partial thromboplastin time) between 1.5 and 2.5, plus 5 mg of warfarin. From the fourth day onwards, their warfarin doses were adjusted using International Normalized Ratios (INR; target range: 2 to 3). This prospective cohort was compared with a historical series of 110 patients had been given 10 mg of warfarin on the first 2 days and 5 mg on the third day with adjustments based on INR thereafter. Outcomes analyzed were as follows: recurrence of thromboembolism, bleeding events and time taken to enter the therapeutic range. Results: Efficacy, safety and length of hospital stay were similar in both samples. The sample that were given 10 mg entered the therapeutic range earlier (means: 4.5 days vs. 5.8 days), were on lower doses at discharge and had better therapeutic indicators at the first return appointment. Conclusions: The 10 mg dosage regimen took less time to attain the therapeutic range and was associated with lower warfarin doses at discharge and better INR at first out-patients follow-up visit.Keywords: anticoagulants; warfarin; dose-response drug relationship; thrombosis; treatment outcomes. ResumoContexto: A melhor dose para o início do tratamento anticoagulante com varfarina vem sendo debatida nos últimos dez anos. Em nosso meio, não observamos nenhum estudo comparativo quanto a estas características. Objetivo: Comparar segurança e eficácia de dois esquemas de dosagem inicial de varfarina para tratamento anticoagulante. Métodos: Foram estudados prospectivamente 110 pacientes de ambos os sexos, consecutivos, com indicação de anticoagulação por tromboembolismo venoso ou arterial. Durante os três primeiros dias de tratamento, estes pacientes receberam doses adequadas de heparina (RT -razão dos tempos -alvo entre 1,5 e 2,5) e 5 mg de varfarina, cuja dose foi reajustada a partir do quarto dia pelo Razão Normatizada Internacional -RNI (alvo entre 2 e 3). Esse grupo foi comparado com série histórica de 110 pacientes que receberam 10 mg nos dois primeiros dias, 5 mg a partir do terceiro dia, com ajuste posterior de dose baseado no RNI. Os desfechos foram: recorrência do tromboembolismo, sangramentos e tempo para alcançar níveis terapêuticos. Resultados: A eficácia, a segurança e o tempo de internação foram similares entre os grupos. O grupo que recebeu 10 mg atingiu níveis terapêuticos mais precocemente (média de 4,5 dias × 5,8 dias), sendo as doses na alta menores e ...