Warfarin anticoagulation data from forty-seven patients studied retrospectively was used to develop an algorithm for predicting daily maintenance dose of warfarin. A plot of prothrombin time versus cumulative warfarin loading dose was made for each of the forty-seven patients and the aera under the curve (AREA) measured from the baseline prothrombin time to a prothrombin time of 20 seconds. A linear correlation between the established daily maintenance dose of warfarin and AREA was derived regression analysis: Daily maintenance dose (mg/day) = 0.0465 x (AREA) + 1.5. The correlation was then used to predict daily maintenance dose in twenty-four patients studied prospectively. The mean prothrombin time for a seven day stabilization period after loading for all patients in the prospective study was 21.5 +/- 2.2 seconds and the seven day mean prothrombin time for each patient fell between 19 and 24 seconds. The results of the prospective study indicate that this technic is useful in the early, accurate prediction of a daily maintenance dose of warfarin.