Dual-wavelength radar studies aimed at the detection of hail generally assume that the two radars used are sampling identically the same volume in space. In this paper we examine the effects of mismatched antenna beam patterns, both the main lobes and the side lobes, on dual-wavelength hail detection. Mismatched beam patterns produce erroneous hail signals which can be quite misleading. The NCAR CP-2/M33 dual-wavelength radar produced erroneous negative hail signals as large as − 20 dB in some parts of storms, while the new CP-2 dual-wavelength radar produces erroneous positive hail signals as large as + 9 dB. We discuss how well matched the antenna beam patterns must be to produce acceptably low erroneous hail signals. We also discuss the serious implications of these findings on earlier dual-wavelength studies.