We undertook this study to see whether genetically-modified corn (Zea mays L.) hybrids show the same yield response to defoliation as do non-GM hybrids. Three near-isoline hybrids, DKC 6015 (conventional), DKC 6017 (Roundup Ready (RR)), and DKC 6019 (Yield Guard Corn Borer Bt + RR) were defoliated by removing the terminal 1/3rd, 2/3rds, or entire leaf blades at silking (R1) and at the kernel-milk stage (R3). Studies were conducted at Clay Center, NE (irrigated) and at Urbana, IL (rainfed) for three years, 2003-2005. Averaged over hybrids and years, yields of untreated plots were 15.8 Mg ha^-1 in Nebraska and 13.4 Mg ha^-1 in Illinois. Hybrids responded similarly to all defoliation treatments at both locations. In Illinois, removing 1/3rd, 2/3rds, and all of the leaf area at R1 decreased grain yield by 6.4, 34.5, and 99.8 percent, respectively, while the same treatments imposed at R3 decreased yields by 6.0, 26.6, and 65.6 percent. Yield losses from partial leaf removal were somewhat higher in Nebraska, where removing 1/3rd, 2/3rds, and all of the leaf area at R1 decreased yield by 7.2, 42.7, and 99.8 percent, respectively, while similar treatments at R3 decreased yield by 10.0, 33.2, and 66.0 percent. Based on these results, we suggest that yield loss predictions due to defoliation from hail be the same for GM corn hybrids as for non-GM hybrids.