The effectiveness of a pharmacist-managed warfarin anticoagulation clinic in maintaining therapeutic prothrombin times and preventing hospitalizations secondary to inadequate control of anticoagulation was evaluated. Patients who had received warfarin sodium for at least one year before being referred to the anticoagulation clinic were studied using retrospective chart reviews. Clinical pharmacists provided patient education, monitored patients for hemorrhagic and thromboembolic complications, and adjusted warfarin sodium dosage to maintain therapeutic prothrombin times. The patients' primary physicians retained responsibility for overall care and were consulted by pharmacists regarding complications of anticoagulation and patient unreliability. The percentage of patients requiring hospitalization (39% versus 4%) and the percentage of prothrombin times outside the therapeutic range (35.8% versus 14.4%) were significantly higher during the preclinic phase (before referral to the clinic) than during the clinic phase. Eight patients were hospitalized for hemorrhagic complications and four for thromboembolism during the preclinic phase; only one hospitalization for hemorrhage occurred during the clinic phase. The warfarin anticoagulation clinic staffed by specially trained pharmacists provided improved therapy compared with treatment received by patients before their referral to the clinic.