COUMARIN DERIVATIVES are frequently employed for short-term or long-term anticoagulation therapy for thrombo-embolic disorders and disturbances in intravascular clotting. Sudden bleeding, with an alarming alteration in prothrombin activity for no obvious or accountable reason, in individuals who have been stabilized for long periods, has been reported previously. Reports of such occurrences seldom refer to the concomitant use of certain pyrazole compounds (ie, phenylbutazone and oxyphenbutazone) with the coumarin-type anticoagulants. Nor do all manufacturers of these drugs mention the possibility in their brochures. One manufacturer of warfarin sodium mentions phenylbutazone as "an exogenous factor which may be responsible for an increased prothrombin time response". A manufacturer of phenylbutazone and related drugs has indicated that a change in their literature and labeling which adds this precaution to the directions is under consideration by the pertinent federal agency.2 These drugs are not uncommonly used in individuals who have arthritis or gout complicated