New heat flow data on the southeastern United States passive margin show that measured, uncorrected flux averages 49+/-11.8 mW m-2 through old (~175 Ma) oceanic crust. Nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths (1900 m to 4250 m) at which data were collected. With the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises, concave down (decreasing dT/dz with depth) and concave up (increasing dT/dz) sediment thermal gradients can be explained by respective average temperature decreases and increases of 0.1-0.2 K amplitude occurring 28-35 days before the measurements. Thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures, while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at ~49 mW m-2 by decompacting and back stripping the 5-8 km of sediment deposited on the margin since the formation of the underlying oceanic crust. This value agrees with previous measurements made on younger crust in the Blake Ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the Baltimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19.5°C to 24.5°C at the bottom-simulating reflector (BSR) on the Blake Ridge slope. .