The sulfhydryl mesna is increasingly used to protect the bladder and kidney from effects of chemotherapy with ifosfamide and cyclophosphamide. Mesna reacts with reagents on urinary test strips designed to detect ketones. Test-strip results were correlated to sulfhydryl concentrations in 931 urine specimens obtained after infusions of mesna. These data may be used to estimate urinary mesna concentrations at the bedside or to test compliance in outpatients given oral mesna therapy.