Ring chromosome 20 (r[20]) syndrome is characterized by mild to moderate learning disability, behavioural disorders, epilepsy, and various dysmorphic features. Although still considered rare, r(20) syndrome is being increasingly diagnosed. More than 30 cases have been described in the literature since 1976. Here we report an additional case of a 14-year-old male with r(20). He had moderate to severe learning disability and epileptic seizures manifesting at about 18 months of age. During the 13 years' follow-up period he showed intractable epileptic seizures, behavioural disorders, and mild dysmorphological features including microcephaly, strabismus, micrognathia, down-slanting eyelids, and ear abnormalities. Frequent episodes of atypical absence or non-convulsive status associated with electroencephalogram changes were seen in follow-up. He was treated with several classical and new antiepileptic drugs, including intravenous immunoglobulin, corticotropin, and vagal nerve stimulation, with unsuccessful control of seizures. Finally, surgical treatment (corpus callosotomy) was performed at the age of 13 years; severity of tonic seizures was diminished, but frequency was unchanged. Although his behavioural problems, e.g. hyperactivity, were mild in early childhood they became more severe when he was 11 years old. Aggressiveness, compulsiveness with self-injury, and panic attacks developed at the age of 13 years, and were more pronounced after callosotomy. This case report provides the first description of deterioration in psychological situation in patients with r(20) intractable epilepsy. The patient was diagnosed with r(20) syndrome after 13 years of clinical follow-up. Karyotype analysis should, therefore, be performed in every patient with intractable epilepsy of unknown aetiology.