Etodolac (CAS 41340-25-4) is a non-steroidal anti-inflammatory drug used clinically. The bilirubin levels in urine samples from patients treated with etodolac were determined with diagnostic tests (diazo and oxidized methods). The urine samples gave a positive reaction with a diazo method, but not with an oxidized method. The serum concentrations of bilirubin in the patients were also in the normal range. These results indicate that the positive reaction of the urine samples from patients treated with etodolac is false and caused by urinary metabolites of etodolac. To identify the false positive substance in the urine samples, the urinary metabolites of etodolac were extracted with ethyl acetate. The extract was injected into a high performance liquid chromatography (HPLC) column and the eluate was mixed with the diazo reagent. Three positive fractions were found. The retention times of the two metabolites in HPLC matched with those of the 6- and 7-hydroxylated metabolites of etodolac. In addition, authentic compounds of the 6- and 7-hydroxylated metabolites gave a positive reaction in the diazo methods. These results indicate that the 6- and 7-hydroxylated metabolites are mainly responsible for the false positive reactions of the urine sample from patients treated with etodolac.