The geothermal heat flux has been measured at 15 sites in the inlets of southwestern British Columbia by using the ocean probe technique. Corrections have been applied for variations in bottom water temperature, sedimentation, thermal refraction by the sediment prism, topography, warm-rim effect, Pleistocene thermal history, uplift, and erosion. The accuracy of the values is about ¿0.3 μcal cm-2 s-1 (¿13 mW m-2). The relative accuracy between the values is about half of this range. The data indicate a pattern of low heat flow from the coast inland about 200 km to the heads of the inlets (13 values average 0.9 μcal cm-2 s-1 or 37 mW m-2). Stations at the heads of two inlets (1.5 μcal cm-2 s-1 or 63 mW m-2) mark the transition to high heat flow further inland (about 2.0 μcal cm-2 s-1 or 84 mW m-2). From west to east the heat flow transition coincides with the first occurrence of recent volcanic centers and hot springs, the transition from high to low Bouguer gravity, and the transition to low deep crustal and upper mantle electrical resistivity and to recent regional uplift. The coast low heat flow is explained partly by low crustal radioactive heat production and in part by the heat sink effect of the cold oceanic Juan de Fuca plate being subducted along the continental margin. The high inland heat flow probably is produced by the upward convective transport of heat by magma and thinning of the lithosphere that occurs when the sinking plate reaches a depth where temperatures are sufficient for partial melting. Low mantle temperature and thus high density in both the continental lithosphere and the underlying subducted oceanic lithosphere under the coast zone explain the high Bouguer gravity and thick crust in this region compared with the low gravity and thin crust inland. To the north of about 51¿N where at present there is transform fault motion along the continental margin, there probably was subduction prior to about 10 m.y. ago. The gravity and crustal thickness data suggest that the high density in the subducted oceanic lithosphere has disappeared but that upwelling and a hot thin continental lithosphere persist inland of 200 km from the continental edge.