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Context: Albendazole is used in the treatment of cysticercosis and hydatid disease, and its prolonged administration can generate adverse effects attributed to the active metabolite, albendazole sulfoxide (ABZSO), whose measurement requires validated methods that help monitor patients under treatment. Aims: To validate a high-performance liquid chromatography method with mass detection (HPLC-MS) for the determination of ABZSO in human plasma. Methods: HPLC-MS was performed on C18 columns (100 × 4.6 mm × 3.5 µm) and mobile phase composed of methanol, water and ammonia solution (25%) (800:200:0.1 v /v/v) at a flow rate of 0.5 mL/minute. The calibration curve was built with an albendazole standard, and detection in a triple quadrupole system. Results: The method was linear (1-1500 ng/mL, R2 = 0.999) with a detection limit of 1 ng/mL. ABZSO concentrations at 1, 5, 50, 100, 200, 500, 1000, and 1500 ng/mL had relative standard deviations of 10.4, 8.4, 7.8, 2.9, 7.4, 3.2, 3.4 and 1.3%. It was highly precise with a mean coefficient of variation of 1.411%, and accurate with a bias of -14.7; 2.8 and -0.8% in ABZSO concentrations of 5, 500 and 1000 ng/mL. The average recovery was 89.2%, and the stability of ABZSO after refrigeration had an average coefficient of variation of 2.3%. Conclusions: The HPLC-MS method is valid and reliable to determine ABZSO, and could be used in pharmacokinetic, bioequivalence, and other studies that evaluate the biological monitoring of patients receiving oral treatment with albendazole.