This study describes the clinical spectrum of patients with Klinefelter's syndrome and seizures. Klinefelter's syndrome is a sex chromosomal abnormality and the most common cause of male hypogonadism. It is characterized by cognitive dysfunction, hypogonadism, and abnormalities of physical maturation. Neurologic impairment has been recognized, but seizures have received little attention. The authors describe three American patients and discuss nine additional patients from two European centers previously reported with Klinefelter's syndrome and seizures. The most common profile of patients with Klinefelter's syndrome and seizures includes mental retardation, behavior problems, epileptiform electroencephalograms (EEGs), and generalized tonic-clonic seizures. The seizures of six of 11 patients with epilepsy were well controlled with antiepileptic drugs. One patient had a single seizure and was not treated with medication. In patients with Klinefelter's syndrome and recurrent seizures, the electroclinical spectrum is heterogenous and outcome with antiepileptic drug treatment is favorable.