The invention relates to the fluid treatment, such as acid stimulation, of a subsea hydrocarbon well via a subsea wellhead/Christmas tree. Fluid is delivered directly to the subsea wellhead from a pumping vessel. Control of the delivery of fluid is from the pumping vessel via a fail-safe close valve in the delivery line. The Christmas tree subsea module is controlled directly from a host platform via a subsea cable, whilst a radio data link between the vessel and host platform provides communication of downhole data to the pumping vessel during the operation.