A procedure has been obtained for calculating the effect of cooling on turbine-stage efficiency by visualizing the flow in the blade passages as one-dimensional compressible flow in a conical tube with heat transfer and friction. The use of Reynolds’ analogy between friction and heat transfer permits the results to be correlated in terms of the blade-profile loss coefficient in low-speed flow, the change of passage area in the blade section and the ratio of average blade-surface temperature to the stagnation temperature of the gas relative to the blades. Simple expressions are obtained for the amount of heat abstracted in a cooled row of blades and the drop in turbine-stage efficiency resulting from cooling. Calculations for some typical stages show that, with blades cooled appreciably below the gas temperature, the amount of heat removed may be as much as 5 per cent of the calorific value of the fuel per row of cooled blades and the decrease in turbine-stage efficiency as much as 3 per cent when both nozzles and blades are cooled appreciably. The use of impulse and low reaction stages reduces to a small fraction of a per cent the effect on the stage efficiency of cooling the rotor blades only.