An increased interest in the persistence of attention-deficit hyperactivity disorder (ADHD) into adulthood brings concern that adult ADHD may become overdiagnosed. As with ADHD in childhood, various other adult psychological and medical disorders may be associated with ADHD in adulthood, and these related disorders may be mislabeled and mistreated as simple ADHD. One such medical condition is sleep apnea. This paper reviews similarities and differences between ADHD and sleep apnea and presents case histories of six adult patients diagnosed with ADHD who were subsequently found to have at least comorbid obstructive sleep apnea (OSA). Clinicians who diagnose and treat patients with ADHD are advised to become familiar with sleep apnea as a life-threatening condition that presents with overlapping symptoms.