Carcass measures from 970 cattle were collected at the Iowa State University Rhodes and McNay research farms over a 6-year period. Data were from bull and steer progeny of composite, Angus, and Simmental sires mated to three composite lines of dams. The objective of this study was to evaluate effects of sex differences on genetic parameter estimates for carcass traits. Estimation of genetic parameters using a threetrait mixed model showed differences between bulls and steers in h2 and genetic correlations. Heritability for carcass weight, percentage retail product, retail product weight, fat thickness, and longissimus muscle area from bull data were .43, .04, .46, .05, and .21, respectively. The corresponding values for steer data were in order of .32, .24, .40, .42, and .07, respectively. Furthermore, genetic correlations between some of the traits were different both in magnitude and direction when evaluated by sex. The results suggested possible differences in genetic parameter estimates between bulls and steer data. Hence, further study on effects of sex differences on genetic parameter estimation and a designing possible strategy to overcome the problem were emphasized.