Hydrocarbon exploration wells provide sufficient information to analyse the present-day thermal regime in the Celtic Sea basins. This information consists of bottom hole temperatures (BHTs), geophysical well logs, composite logs and rock cuttings from the major formations. The BHTs provide numerous but low-quality data which require extensive processing before they provide reliable estimates of formation temperature. Standard corrections (Horner plots) to multiple BHTs can be modified to correct single EHT measurements. A least-squares inversion based on a thermal resistance (Bullard) model for conductive heat flow can map many noisy Horner-corrected BHTs into a set of formation temperature estimates with relatively small errors. The average geothermal gradient is 32 degrees C km(-1). Laboratory measurements of the thermal conductivity of rock cuttings taken from representative formations in selected wells give matrix conductivities at room temperature. These sample measurements are combined with information about formation porosity and temperature to give in situ thermal conductivities for each formation in each well. These are then used with formation temperature gradients from the BHT analysis to estimate heat flow. The heat flow obtained for the Celtic Sea basins varies between 59 and 81 mW m(-2). The highest values appear to be where sediments are thickest.