Thermal gradients of 24¿-36 ¿C/km have been found in the slope cover sediments of the accretionary prism off northeast Japan (Deep Sea Drilling Project (DSDP) sites 438A, 439, and 440). These gradients lead to the prediction of temperatures in excess of 400 ¿C at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity. Such temperatures conflict with the predictions of temperatures of 150 ¿C at even greater depths from conductive thermal models and petrologic studies of low-temperature blueschists at similar ancient margins. One-dimensional, steady state conduction and advection via Darcian water flow, of heat, up through the prism was simulated numerically. The measured thermal gradients can be reconciled with temperatures less than 125 ¿C at the prism base by water flowing upward at rates of the order of 10-10 m/s. Such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast Japan prism would flow up through a <10 km width of the prism (perpendicular to the bench) and would require the DSDP sites to have been located fortuitously over the localized zone(s) of water flow. Smaller velocities will reproduce the measured gradients, and thus greater widths through which water flow occurs are predicted, if the temperature at the prism base is higher and/or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate. Where water flow is significant, as likely wherever abundant, water-rich sediments are subducted, thermal gradients below a few kilometers in accretionary prisms may be much lower than near-surface values. This has profound implications for the thermal structure of the prism off northeast Japan and elsewhere. ¿American Geophysical Union 1987