Steady-state plasma levels of warfarin were measured in thirty-nine ambulatory patients attending a haematology clinic. 2. Plasma concentrations, plasma clearance, and prothrombin ratio showed no significant differences when patients below 65 years of age were compared with those above 65 years. The mean daily maintenance dose of warfarin required was marginally greater for the younger patients but the difference was not statistically significant (P = 0.10). 3. A significant relationship was found between the daily maintenance dose of warfarin required by all patients and their mean steady-state plasma level (r = 0.5667, P less than 0.001). 4. For one elderly hospitalized patient it was apparent that there was a marked depression of the vitamin K-dependent clotting factor synthesis by warfarin which was associated with a hypoprothrombinaemic response.
