Atomoxetine (ATX) is a potent inhibitor of the noradrenaline reuptake transporter approved since 2002 for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children, adolescents, and adults as alternative treatment to methylphenidate. A procedure based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been developed for the determination of ATX and its main metabolites (4-hydroxyatomoxetine - 4 hydroxyATX - and N-desmethylatomoxetine - des-methylATX) in hair of one treated child and five treated adolescents. Since hair samples can be easily collected without the need for specials skills and exposing a patient to discomfort, hair testing of ATX and eventually of its metabolites should be useful, especially in case of pediatric patients, to check compliance in a wider time-window. After addition of duloxetine as internal standard, hair samples were overnight digested with 2. ml 1. M NaOH at 45 °C. Then, analytes were extracted from alkaline solution with two different 2. ml aliquots of tert-butyl methyl ether. Chromatographic separation was achieved at ambient temperature using a reverse-phase column and a mobile phase of 40% of water-60% 5. mM ammonium acetate, 50. mM formic acid, 4. mM trifluoroacetic acid in acetonitrile-water (85:15, v/v). The mass spectrometer was operated in positive ion mode using multiple reaction monitoring. The method was linear over the concentration range 0.2-50. ng/mg hair for the all analytes under investigation, with an intra- and inter-assay imprecision and inaccuracy always less than 20% and an analytical recovery between 33.1% and 76.1%, depending on the considered analyte. Only ATX and 4-hydroxyATX were detected in hair samples with concentrations varying from 0.2 to 2.0. ng/mg hair and from 0.3 to 1.0. ng/mg, respectively. Notwithstanding the absence of any dose-hair concentration relationship, hair monitoring of ATX and concomitant medications commonly administrated in ADHD children and adolescents can be crucial in verifying long-term compliance to prescribed medication in individuals displaying a non negligible tendency to refuse drugs and to lie on the adherence to therapy as a specific symptom of the disease. © 2011 Elsevier Ireland Ltd.