The diagnosis of attention-deficit hyperactivity disorder (ADHD) in adulthood has become increasingly frequent. Carelessness, anxiety and depression are emphasized as clinical symptoms of adult ADHD. However, similar symptoms are common in other mental and physical disorders, such as mood disorders, anxiety disorder, schizophrenia and early-onset dementia. In this report, we present a case of a man in his early 50 s with early-onset Alzheimer's disease (EOAD) who was initially diagnosed with ADHD. He had been prescribed atomoxetine for carelessness, such as missing meeting details and based on medical interviews at a psychiatric clinic. However, his carelessness deteriorated, and agitation emerged despite his medication. At a subsequent interview, methylphenidate was added. At two months after starting methylphenidate, he suffered myocardial infarction (MI). Subsequently, atomoxetine and methylphenidate were discontinued. The patient successfully recovered from the MI, but his symptoms of carelessness and agitation remained. He was referred to our hospital for a further examination, including an evaluation for physical disorders that induce psychiatric conditions. On brain magnetic resonance imaging, he showed no evident atrophy of the cerebral cortex or hippocampus. Technetium-99m ethyl cysteinate dimer single photon emission computed tomography (ECD-SPECT) showed diffuse hypoperfusion of the parietal and occipital lobes (posterior association area). In cognitive function tests, the Mini-Mental State Examination (MMSE) was 21/30, and the Wechsler Memory Scale-Revised (WMS-R) showed obvious memory deficit. Furthermore, a detailed interview showed that, during childhood, he'd shown no symptoms of carelessness or hyperactivity symptoms other than losing items, and no carelessness or hyperactivity symptoms had been observed throughout adulthood until 50 years old. As a result of these examinations and interviews, he was diagnosed with EOAD. This case suggests that when diagnosing ADHD and medicating ADHD symptoms in adulthood, it is necessary to pay close attention to its distinction and coexistence with other mental and physical disorders, including dementia. In addition, a detailed medical interview concerning the patient's development and growth history, cognitive function tests, and neuroimaging tests are considered useful tools for making a differential diagnosis.
 Authors' abstract