The terminal deletion of the long arm of chromosome 18 is relatively common among cytogenetic abnormalities, which occur incidentally in approximately 1 in 40,000 live births. Proximal interstitial deletions of the long arm of chromosome 18 are less frequent. The critical region on chromosome 18 of this syndrome is del(18)(q12.2q21.1) and has recently been recognized as a new clinical entity. We describe a 8-year-old boy with developmental delay, obesity, and epilepsy, with characteristic facial anomalies in whom G-banding chromosome analysis revealed a unique karyotype of 46, XY, del(18)(q12.2q21.1). The patient was diagnosed with interstitial deletion chromosome 18q-syndrome. He received weight control therapy from a medical dietitian. For his epilepsy, he was administered oral carbamazepine at 4 mg/kg/day. At age six, he entered a special needs elementary school. After entering school, he often showed hyperkinesis, and was diagnosed with attention deficit hyperactivity disorder with mild mental retardation. Because patients with only del(18)(q12.2q21.1) have no serious associated malformations, physicians should be aware that even adult patients may have 18q-syndrome. Therefore, if epilepsy occurs in patients with minor facial anomalies, psychomotor retardation, obesity, and the possibility of 18q-syndrome with del(18)(q12.2q21.1) should be kept in mind, and chromosome testing should be performed.