Urologists use intravenous dyes in diagnosing genitourinary fistulas and in investigating ureteral patency. Methylene blue and indigo carmine are the most common dyes used today. Generally, patients with clinically normal renal function demonstrate dye in their urine after several minutes. We report on 2 patients in whom methylene blue was not visualized after intravenous injection. A review of urologic and pharmacologic published reports led to a possible explanation for this phenomenon. Methylene blue can metabolize into leukomethylene blue, which is colorless in urine. Indigo carmine, however, is not readily metabolized but is rather freely filterable by the kidneys. Therefore, it is important to appreciate that nonvisualization of methylene blue may be a metabolism effect and not an anatomic one.