The peak field that is radiated by a distant lightning return stroke almost always occurs within a few microseconds after the onset of the stroke, and therefore this peak is produced at a time when most of the stroke current is still close to the ground. Estimates of the peak electromagnetic (EM) power radiated by return strokes have been made by integrating the Poynting vector of measured fields over an imaginary hemispherical surface that is centered on the lightning source assuming that ground losses are negligible. Values of the peak EM power from first and subsequent strokes have means and standard deviations of 2+2¿109 W, respectively. The average EM power that is radiated by subsequent strokes, at the time of the field peak, is about 2 orders of magnitude larger than the optical power that is radiated by these strokes in the wavelength interval from 0.4 to 1.1 μm; hence an upper limit to the radiative efficiency of a subsequent stroke is of the order of 1% or less at this time. From the above values of the radiated power, we also infer that at the time of the initial current peak (1) the total voltage drop on the high-current portion of a return stroke must be at least 2 to 7¿103 V in first strokes and 2 to 4¿103 V in subsequent strokes, (2), the total resistance of the high-current channel must be at least 6 to 20 &OHgr; in first strokes and 9 to 35 &OHgr; in subsequent strokes, and (3) the energy that is required to form the propagating tip of the channel must be at least 102 to 103 J/m.