This paper reports high-accuracy measurements of geocoronal Balmer &agr; line profiles and demonstrates that the profiles are well fit with a model which includes cascade excitation by solar Lyman series radiation from n>3 in addition to the direct excitation of n=3 by solar Lyman &bgr;. The increase in the signal-to-noise of our data is made possible by the use of the Fabry-Perot annular summing technique implemented at our Fabry-Perot facility at the University of Wisconsin's Pine Bluff Observatory. The new sensitivity has allowed us to make a detailed examination of line profile asymmetries and to conclude that they are compatible with predictions that of the order of 10% of the geocoronal Balmer &agr; emission is caused by the cascade process. Cascade excitation alters the observed profile because it produces Balmer &agr; emission along fine structure paths yielding slightly shifted wavelengths not present in direct Lyman &bgr; excitation, which is the predominant excitation mechanism for geocoronal Balmer &agr;. We discuss how fine structure excitation affects studies of non-Maxwellian exospheric hydrogen velocity distributions and effective temperatures through Balmer &agr; line profile measurements. In a broader context, we consider how inclusion of the cascade excited emission in future radiation models can enhance their accuracy and their potential for assisting in the isolation in the data of shorter-term solar geophysical effects and longer timescale changes in exospheric hydrogen densities. ¿ 1998 American Geophysical Union