The objective of this study was to characterize the populationstructure and genetic diversity of registered American Red Anguscattle. Inbreeding and average relationship coefficients, effectivepopulation size, effective number of founders, and effectivenumber of herds supplying grandparents to the population werecalculated from the recorded pedigree. Inbreeding in 1960 was10.7% and decreased until 1974 at a rate of 0.2% per year, whereasin 1975 inbreeding was 3.2% and increased until 2005 at a rateof 0.02% per year. The numerator relationship coefficients ofthe 10 individual paternal grandsires (PGS; sires of sires),paternal granddams (PGD; dams of sires), maternal grandsires(MGS; sires of dams), and maternal granddams (MGD; dams of dams)that had the greatest number of registered grandprogeny, withall other registered animals, increased with their birth yearfrom 1960 on. Average numerator relationships of these withall other PGS, PGD, MGS, MGD, bulls, and sires were greaterfor paternal (PGS, PGD) than maternal (MGS, MGD) pathways. Theeffective population size was 445, with 649 effective founders.The effective numbers of herds supplying PGS, PGD, MGS, andMGD were 435, 369, 453, and 459, respectively. Inbreeding isat a low level and the effective population size is large. Theeffective number of founders and effective number of herds supplyinggrandparents is small in relation to the total number of animalsand herds, indicating the disproportionate influence of a fewfounders and herds on the genetics of the breed. The calculatedparameters indicate satisfactory genetic diversity in AmericanRed Angus cattle.