To determine the best way to reverse the excessive effect of regular anticoagulation in patients with INR > 5 and no bleeding complications, 23 patients with INR > 5 were randomly subdivided into two groups: group A (n = 12) discontinued warfarin for one day and group B (n = 11) received 2 mg of vitamin K1 orally in addition to the usual warfarin dose. INR was determined after 24 h (day 1), after which both groups continued with their usual dose of warfarin. After 48 h (day 2), warfarin dosage was changed according to the INR value. On day 9, INR values were determined again. Five out of twelve patients in group A had INR values > 5 on day 1. One patient in group A had an INR value < 5 both on days 1 and 2. All eleven patients in group B had INR values < 5 on day 1, and all but one on day 2. On day 9, INR values were acceptable (INR 2.0-4.5) in ten group A patients and eight group B patients. These findings suggest that a low oral dose of vitamin K1 is a convenient treatment for excessive anticoagulation in patients with no bleeding complications.