BACKGROUND: Seasonality characterizing influenza epidemics suggests susceptibility to climate variation. El Nino southern oscillation (ENSO), which involves two extreme events, El Nino and La Nina, is well-known for its large effects on inter-annual climate variability. The influence of ENSO on several diseases has been described. OBJECTIVES: In this study, we attempt to analyze the possible influence of ENSO on the timing of the annual influenza activity peak using influenza-like illness report data in Japan during 1983-2007. MATERIALS: Influenza surveillance data for 25 influenza epidemics, available under the National Epidemiological Surveillance of the Infectious Diseases, was used in this study. ENSO data were obtained from the Japan Meteorological Agency. RESULTS: Influenza-like illness peak week varied largely during the study period, ranging between 4th and 11th weeks (middle of winter to early spring). The average of peak week during ENSO cycles (n = 11, average = 4.5 +/- 0.9) was significantly earlier than in non-ENSO years (n = 14, average = 7.6 +/- 2.9; P = 0.01), but there was no significant difference in the peak timing between hot (El Nino) and cold (La Nina) phases. Earlier peaks of influenza activity were observed in 16, out of 25, epidemics. These coincided with 10 (90.9%) out of 11 ENSO and 6 (85.7%) out of seven large-scale epidemics. CONCLUSION: Influenza activity peak occurred earlier in years associated with ENSO and/or large scale epidemics.