Children and adolescents in particular are often misdiagnosed with "ADD" and "Bipolar Disorder," the comorbidity of which is certainly theoretically possible, but which in actuality is much more rare than the vastnumbers of young people who carry both diagnoses.
These conditions are often confused for one another, although mania and hyperactivity are quite distinct syndromes, but the difficulty--or inability--of a clinician to properly distinguish them is likely the reason so many mental health providers default to applying both labels, despite the exceedingly low likelihood that a child or teenager is truly affected by, and simultaneously presenting with, both conditions. Bipolar Disorder affects approximately 1% of the population, but infrequently presents before the second decade of life, and is often not formally diagnosed until after several bouts of major depression, which tend to precede the first manic episodes in many patients with this condition, whereas bonafide ADD is always evident by the time a child begins school, and toddlers and preschool-age children with the hyperactive type of ADD (ADHD) are obviously unlike their unaffected siblings and peers.
The lifetime incidence of persons affected with Bipolar Disorder and ADD would be a minor fraction of 1% of the population, but the prevalence of both conditions in children and adolescents would be even less--far less--because individuals rarely exhibit frank mania at very young ages, even if they truly are affected by a Bipolar-Spectrum illness. And yet, I have seen vast numbers of children labeled with these dual diagnoses; in hospitalized populations, for example, the apparent prevalence of "Bipolar Disorder and ADHD" in children is orders of magnitude greater than what would be expected.
A child or teenager carrying both of these diagnoses has virtually invariably been misdiagnosed with one or the other condition, and too often, both diagnoses are wrong. The difficulty with diagnosing both conditions in children (again, almost always erroneously), is the dilemma diagnosing both conditions gives rise to, with regard to treatment options. The concurrent use of stimulants and mood stabilizers in a single patient is rarely justified, particularly in children, but I often see medication regimens that are treating diagnoses, instead of actual, observable, and objective, target symptoms.