A study of 31 patients with cardiovascular disease who were taking warfarin regularly had shown pronounced intersubject differences in serum protein binding of warfarin and a highly significant correlation between the body clearance of warfarin and the free fraction of the drug in serum. Similar observations have been made in experimental animals and are consistent with predictions based on theoretical considerations. The purpose of this investigation was to determine the intrasubject variation in the free fraction of warfarin in serum. Samples of serum were obtained from 23 of the 31 patients previously studied. The time interval between the two studies was 3.4 to 5.7 mo. The daily dose of warfarin had been changed by 10.6% on the average. With two exceptions, there was no change in concurrent medications. The ratio of free fraction values of warfarin in serum, second/first study, was 0.948 ± 0.297 (mean ± S.D.), and there was a highly significant correlation (p < 0.001) between the individual free fraction values in the first and second studies.