One hundred and ten closely spaced probe heat flow measurements provide new constraints on the thermal regime of the northern Cascadia accretionary sedimentary prism off Vancouver Island. Complementary heat flow values have been obtained from the depth of a bottom-simulating seismic reflector (BSR) that is interpreted to mark the thermally controlled base of a methane hydrate layer. The only local heat flow variations observed are associated with a sediment slump that is seen in SeaMARC II acoustic images and with the outcrop of several major thrust faults. These anomalies appear to be the result of material displacement only; strong localized fluid flow along faults or elswhere is not evident in the probe or BSR results. Fluid expulsion resulting from the dewatering of the prism sediments appears to occur regionally in the 10–20-km-wide zone landward of the deformation front. In this area there is a significant disagreement between the probe and BSR heat flow estimates (roughly 30%) that can be explained by a regionally uniform vertical fluid flow at a rate of about 8×10−10 m s−1. This is in good agreement with the estimated fluid expulsion rate required by the decrease in porosity landward of the deformation front, as estimated from the increase in seismic velocities derived from multichannel reflection data. The heat flow in Cascadia Basin seaward of the deformation front, corrected for the effect of sedimentation, is in excellent agreement with that predicted by cooling plate models. Landward, there is a regional trend of decreasing heat flow across the accretionary prism, which is consistent with a model of simple tectonic thickening. Temperatures at the top of the oceanic crust are estimated to be over 200°C beneath the 2–3 km of sediment in Cascadia Basin. Temperatures at the interface between the prism and the oceanic crust continue to increase landward, and reach 400°–450°C beneath the middle to inner continental shelf. Initiation of megathrust earthquake failure along the main subduction thrust may be thus restricted by the high temperatures to the zone beneath the continental slope and outer shelf. Early and shallow metamorphism of the prism material probably is another important consequence of the early deep burial of the young oceanic crust in this setting.