OBJECTIVES: Effective use of over-the-counter (OTC) medications depends on purchasers' knowledge of their indications. This study examines consumer knowledge regarding the urinary tract analgesic phenazopyridine, which recently became available in the United States without prescription. METHODS: We conducted a cross-sectional survey of a stratified cluster random sample of purchasers of OTC phenazopyridine (N = 434) in 31 Los Angeles retail pharmacies. RESULTS: The response rate was 58%. Only 29% correctly characterized the likely cause of their symptoms, and only 57% correctly characterized the action of the drug. Worse consumer knowledge was associated with nonwhite race, first-time use, and less contact with health providers. CONCLUSIONS: Many consumers possess poor knowledge about phenazopyridine, potentially leading to undertreatment, especially in vulnerable populations without access to care.