Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder encompassing inattentive and/or hyperactive/impulsive symptoms that must be present before age 12. Not all patients presenting with attentional/cognitive deficits and/or hyperactivity have ADHD. In this review, the author discusses several medical conditions that may present with symptoms resembling those seen in patients with ADHD, including seizure disorders, diabetes mellitus, thyroid dysfunction, sleep deprivation, sleep disordered breathing, traumatic brain injury, inflammatory bowel disease, iron deficiency, and anemia (Table). In addition to the medical conditions that may present with symptoms similar to those in ADHD, there are also psychiatric conditions (e.g., mood disorders) and additional neurological conditions (e.g., early phase dementia) that may present with cognitive impairments similar to those seen in ADHD. The author suggests that the differential diagnosis of ADHD must include a complete medical (as well as psychiatric) work up and evaluation to exclude non-ADHD causes for cognitive dysfunction (including attentional issues) and/or hyperactivity/impulsivity. Furthermore, it is recommended that the DSM-6 include an exclusion in the diagnosis of ADHD if symptoms are attributable to physiological effects of a substance or another medical condition (as is done for e.g., mood disorders). This is especially important given that the misdiagnosis of ADHD, as well as consequent treatment with ADHD medications, may lead to poorer (and in some cases, dire) patient outcomes.