The guaiac-impregnated test slide (Hemoccult) used in testing for occult blood in the stool gave a false-positive reaction in a trauma patient when a povidone-iodine antiseptic solution (Betadine) was used prior to the insertion of a urinary catheter. Laboratory testing using serial dilutions of povidone-iodine solutions indicates that as little as 0.005 mL of a 1:1,000 dilution will give a positive guaiac reaction. Separate testing confirms that the iodine component is responsible for this false-positive reaction. The mechanism is probably direct oxidation of the alpha-guaiaconic acid chromogen. Patients with perianal decubiti who are being treated with povidone-iodine solution, or those who have recently had a urinary catheter inserted, may need to have the area washed prior to guaiac testing. Antiseptics not containing iodine may be used in trauma patients to prevent false-positive guaiac reactions.