We studied the relation among serum cytokine levels, EEG changes, and mild neurological complications (delirium and febrile seizure) in children with influenza. The serum levels of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and soluble tumor necrosis factor receptor-1 (sTNFR-1) were measured in 27 children with proven influenza infection with mild neurological complications (10 patients with delirium and 17 with febrile seizures) and seven control children. EEG was recorded in 14 children with neurological complications. EEG showed focal slowing in four of nine patients with delirium and in four of five with febrile seizures. Generalized slowing was observed in one patient with delirium. The median serum IL-6 level was 31.2+/-15.1 pg/ml (range, 7.5-64.5 pg/ml) in the delirium group, 42.3+/-44.0 pg/ml (range, 8.0-196.0 pg/ml) in the febrile seizure group, and 15.4+/-7.0 pg/ml (range, 7.2-28.0 pg/ml) in the control group. Serum TNF-alpha and sTNFR-1 levels were not different among three groups. Mild neurological complications associated with influenza were related to the mildly abnormal serum IL-6 levels and EEG findings. The combination of these parameters will be useful for early diagnosis and differentiation of neurological complications in children with influenza. Further studies will be necessary for investigating that IL-6 has the diagnostic value for differentiation between severe encephalopathy and mild neurological complications in children with influenza.