Subjects will be enrolled via informed permission/assent and will be studied as outpatients utilizing resources provided by the Pediatric Clinical Research Unit located in the Division of Clinical Pharmacology and Medical Toxicology at Children's Mercy Hospital. For the ADHD group, patients being treated with atomoxetine at the time of study enrollment will be allowed to participate. Although in vitro studies indicate that atomoxetine inhibits CYP2D6 activity by ~50%, there does not appear to be any effect in vivo as single dose pharmacokinetics of the CYP2D6 substrate, desipramine, were unaffected by four days of atomoxetine 60 mg b.i.d. Thus, we do not expect any substantial effect on DM biotransformation. Furthermore, if during the six month interval between phenotyping visits ADHD patients are prescribed medications that are known CYP2D6 inhibitors, they will be allowed to continue in the study; the dose and start date of these medications will be recorded and the effect of the inhibitor on CYP2D6 phenotype (urinary metabolite ratio and endogenous biomarker) will be determined. These data points will be excluded from the ontogeny and genotype-phenotype correlation analysis