A fast and selective liquid chromatography–tandem mass spectrometric (LC/MS/MS) method for the screening of 18 diuretics and probenecid in human urine is presented. Analyses were performed on a LCQ-Deca® instrument equipped with ESI-interface using scan by scan polarity changing. All diuretics and probenecid were separated in less than 20 min after liquid–liquid extraction with ethyl acetate. The LOD for all substances was 100 ng/mL or better. The method was applied to detect diuretics after the oral administration of several drugs including hydrochlorothiazide, bumetanide, spironolactone, furosemide, amiloride, triamterene, chlortalidone and epithizide. All diuretics could be detected for periods up to 96 h after the intake of therapeutic amounts. Copyright © 2002 John Wiley & Sons, Ltd.