Although the interactions of lightning with aircraft and many other structures are critically dependent on the amplitudes of the electromagnetic fields in the 1 to 20 MHz frequency interval, there are relatively few measurements of these spectra in the lightning literature. In this paper, we present amplitude spectra of the fast-rising, initial portion of fields that are produced by return strokes, leader steps, and cloud pulses. These data have been derived from ground-based measurements of dE/dt signatures that were recorded in the time-domain on a triggered oscilloscope. The spectral amplitudes of first return strokes match previously published data at 1 MHz and decrease with increasing frequency as 1/ƒ between 1 and about 6 MHz and as 1/ƒ2 between 6 and 20 MHz. The spectral amplitudes of leader steps just before return strokes and the fast portion of cloud pulses that triggered the recording system tend to lie 5 to 10 dB below the amplitudes of first return strokes over the entire frequency interval.