This study investigated the clinical and EEG characteristics of initial status epilepticus (SE) during infancy in patients with mesial temporal lobe epilepsy (MTLE). The subjects were six patients who had been brought to our emergency clinic and treated for their initial SE between 1977 and 1988, and later developed MTLE. We reviewed the medical records and laboratory findings at the time of the initial SE, and the clinical evolution up to the development of MTLE. The six patients included four females and two males. The initial SE developed at ages ranging from 7 months to 2 years and 9 months with a mean of 1 year and 2 months. These episodes were characterized by an elevated temperature of more than 38 degrees C (4/6 cases), clusters of prolonged seizures during one episode of SE (4/6 cases), long-lasting SE (120-380 min, mean 227 min, 6/6 cases), postictal prolonged loss of consciousness (median 5 h, 6/6 cases), and the presence of Todd's paralysis (3/6 cases). The lateralization of the ictal or postictal EEGs of the SE in five of the six cases was identical to that of the hippocampal atrophy later confirmed by MRI. Follow-up EEG examinations at a 6 month interval demonstrated temporal spike discharges appearing only after the onset of complex partial seizures. Two patients, who had no fever at the initial SE, were characterized by a very early appearance of epileptic EEG abnormality and a short interval between the initial SE and the development of complex partial seizures, suggesting that the SE was the first epileptic manifestation. The result of this study showed that SE progressing to MTLE tends to have complicated clinical manifestations characterized by clusters of unilateral or generalized SE followed by prolonged postictal unconsciousness, generalized clinical manifestations despite lateralized ictal EEG discharges, and the Todd's paresis in addition to the prolonged seizure duration.