We studied the length of stay needed to record a certain number of seizures in ahighly selected group of patients with intractable epilepsy in the final stages of presurgical monitoring. The mean length of stay needed to record one seizure was 2.9 to 3.7 days, depending on the recording technique, 4.5 to 5.5 monitoring days to record three seizures, and 6.1 to 7.6 days to record five seizures. It took 5 days to record at least one seizure in 90% of patients, 7 to 10 days to record three seizures, and 8 to 12 daysto record five seizures. One-third of all patients had fewer than five seizures during their stay. Extrapolating from the results in our unit, we computed a need for 91 to 227 presurgical epilepsy monitoring beds to evaluate 2,000 to 5,000 similar epilepsy surgery candidates requiring invasive monitoring in the United States each year. Additional beds would be needed to monitor patients admitted for other purposes.