Hair samples were collected from 17 children (7-16 years of age) who had received methylphenidate therapy on a long-term basis. The dose was 10-60 mg per day. To 20 mg of pulverized hair, phosphate buffer and internal standard (norpethidine) were added. Extraction was performed by ultrasonic treatment, followed by isolation by alkaline extraction with isohexane. Derivatization with N-Methyl-bisheptafluorobutyric amide enabled gas chromatography-mass spectrometry analysis of the perfluorobutyryl derivatives in selected ion monitoring mode. The methylphenidate concentrations lay between 0.073 and 1.1 ng/mg hair. It was noted that at higher daily doses there was a tendency toward increased concentrations in the hair, but for testing of compliance, the incorporation into hair did not occur with the required high correlation between daily dose and concentration in the hair.