Steers from research breeding projects (n=404) were serially scanned using Real-Time Ultrasound (RTU) at 30-day intervals from re-implant time until slaughter. Cattle were evaluated for rump fat depth, longissimus muscle area (REA), 12th rib fat thickness (FAT), and percent intramuscular fat (PFAT) to determine the predictability of carcass composition at extended periods before slaughter. Additional background information on the cattle, such as weight, gain, breed of sire, implant and frame score was also utilized. Carcass data was collected by trained personnel at "chain speed," and samples of the 12th rib longissimus muscle were taken for ether extract analysis to determine PFAT estimates. Simple correlation coefficients showed moderately high positive relationship between RTU measures taken less than 7 days before slaughter and carcass measures: REA (r=.66); FAT (r=.74); and PFAT (r=.61). Correlation coefficients for RTU measures taken 96-105 days before harvest and carcass values were: REA (r=.52); FAT (r=.58); and PFAT (r=.63). Regression equations were then developed for the carcass measurements; 46% of the variation could be explained for REA, 44% of the rib fat and 46% of marbling at re-implant time. Age of the animal (p < .01) and frame score (p < .0001) were significant predictors of carcass ribeye area. Rump fat (p<.0001) and PFAT (p<.01) were important predictors of carcass 12th rib fat. Average daily gain was also important in the prediction of carcass marbling score (p<.01). Additional regressions at 61-69 days and 27-41 days before slaughter showed similar results, with R-squared values logically explaining more of the variation closer to the harvest date. Live ultrasound measure is a viable option for assessing carcass composition at re-implant time and predicting final quality and yield grades.