Plasma warfarin half-life was estimated in four patients with renal dysfunction and five normal control subjects. Hypoprothrombinemic responsiveness to a single oral dose of warfarin (0.75 mg/kg) was also evaluated for both groups. The mean warfarin half-life of 29.9+/-5.0 (S.E.M.) hours for renal patients was significantly shorter than the 44.8+/-6.0 hours half-life for normal controls (P less than 0.05). Additionally, a positive correlation was found between warfarin half-life and creatinine clearance. Pharmacologic responsiveness to warfarin was comparable for both groups. It therefore appears that patients with renal dysfunction do not possess an increased susceptibility, either pharmacokinetic or pharmacologic, to the hypoprothrombinemic effect of warfarin.