We investigate the relationship between medium-scale traveling ionospheric disturbance (MSTID) and sporadic E (E-s) layer activities in summer nights by analyzing total electron content (TEC) data obtained from a global positioning system (GPS) network in Japan and ionosonde data obtained at Kokubunji, Japan during May-August in 2001-2005. MSTID activity is defined as delta I/($) over bar, where delta I is standard deviation of the TEC perturbations over Kokubunji within 1 h, and 7 is the background TEC. By analyzing nighttime-averaged (19-02 LT) values of MSTID activity and E, layer parameters, we find that the MSTID activity is closely correlated with f(0)E(s) and f(0)E(s) -f(b)E(s). This result suggests that MSTID and the spatial structures of E, layer could be generated by an electro-dynamical coupling process between the Es layer and F region through polarization electric fields. Furthermore, we suggest that the appearance of the E, layer in the summer hemisphere could play an important role in generating MSTIDs in both hemispheres. (C) 2008 Elsevier Ltd. All rights reserved.