Objective: Measurement of urinary fractionated metanephrines represents a first-line test for the biochemicaldiagnosis of pheochromocytoma. The high performance liquid chromatography coupled to electrochemical detection (HPLC–EC) assays used in the routine clinical laboratory can be subjected to analytical interferences by the presence of drugs or their metabolites. In this paper we describe the interference on urinary normetanephrine (uNMN) caused by amoxicillin.