The medical records of six cases of nesidioblastosis were examined to determine the diagnostic approach, treatment, and neurologic sequelae. All six patients were male, and their ages at the onset of the disease ranged from one day to six months (mean 3.36 ± 2.5 mo.). Initial clinical features were seizure, cyanosis, poor feeding, and apnea. Other subsequent symptoms were developmental delay, hyperactivity, and cold sweating. The Birth weight of the neonatal onset group was heavier than the postneonatal onset group (4.4 ± 0.3 vs 3.26 ± 0.04 kg). Before the diagnosis of hyperinsulinism, steroids of ACTH proved effective for seizure control. Initially, hyperinsulinemia (serum insulin greater than 10 microU/ml) was detected in four cases, but another two cases also showed hyperinsulinism by insulin/glucose(I/G) ratio greater than 0.3 during the fasting test. The glucagon response performed in 2 cases, showed normal and partial responses. Euglycemia was obtained by near total pancreatectomy (95% pancreatic resection)without malabsorption or persistent diabetes. In one case, nesidioblastoma coexisted with nesidioblastosis. Developmental delay was noted in three cases. In this group, the mean duration between symptom onset and operation was longer than the group without developmental delay (1.25 ± 0.47 vs 0.38 ± 0.19 yr).