In this study, the authors characterized the clinical and EEG features of adult patients with Down syndrome who were referred, for more than a 10-year period, to the Epilepsy Clinic in the Cork University Hospital. A retrospective audit of the charts of 28 patients with Down syndrome who had an EEG performed in the Cork University Hospital between January 1, 2000, and September 30, 2009, including clinical follow-up, was carried out. Demographics, age at first seizure, seizure types, EEG findings, antiepileptic drugs, psychoactive medications, and seizure control were documented. Seizures most commonly began in the fourth decade of life. Generalized tonic-clonic seizures were the most common type of seizures (15 patients, 54%), followed by complex partial seizures (5 patients, 18%) and generalized myoclonus (4 patients, 15%). A number of patients had more than one type of seizure. Diffuse background slowing was the dominant EEG abnormality seen in 17 patients (60%). Epileptiform activity was present in 5 patients (18%): 2 had focal epileptiform discharges in the frontal regions, 1 in the central region, 1 in the central posterior region bilaterally, and 1 in the temporal regions. The six patients who had poor seizure control were those who had seizure onset from the fourth decade onward and clinically had complex partial seizures in combination with either generalized tonic-clonic or generalized myoclonus; four of them had epileptiform activity on EEG, with two having focal frontal epileptiform discharges: one in the central posterior areas bilaterally and one in the temporal regions. Five of the 28 patients had a normal EEG and 3 of these had a seizure disorder. There was no correlation found between use of psychoactive drugs and increased frequency/poor seizure control in patients in our study. This study documents the variability of clinical and electrophysiologic features in a well-characterized cohort of patients with Down syndrome with adult-onset epilepsy.