A frequent cause of acquired warfarin resistance is drug interaction; however, ingestion of large amounts of vitamin K in food may also be an etiologic factor. A 31-year-old woman on a weight-reducing diet showed evidence of resistance to warfarin sodium therapy. On a regimen of 35 mg of oral warfarin sodium daily, prothrombin time was 14 s (control, 12 s). Pharmacokinetic studies did not reveal any evidence of impaired adsorption or increased catabolism of the drug. The half-life of her oral warfarin was 26 hours (normal, 15 to 56 hours). Although end-organ resistance was not studied fully, a change of her vegetable-rich, weight-reducing diet (vitamin K content, 1,277 microgram) to a regular diet (vitamin K content, 360 microgram) resulted in substantial reduction in her warfarin resistance. We conclude that in patients on vegetable-rich, weight-reducing diets, a relative resistance to warfarin may be secondary to their increased dietary intake of vitamin K.