To the editor: The report of Silver and Bell in the March issue (1) describes a patient in whom Cimetidine appeared to potentiate the hypoprothrombinemic effect of warfarin. We report an additional case of this potentially harmful drug interaction. A 46-year-old white man had coronary artery revascularization in January 1975. He was placed on warfarin sodium (Coumadin®) at a dosage of 7 mg daily. On this dose, his prothrombin time remained stable within 1.7 to 1.9 times control. In September 1978, because of obstruction of both bypass grafts, repeat coronary revascularization was done. He was discharged on Coumadin, 6 mg