The pedigree data of four pig breeds (Czech Large White – ČBU, Czech Landrace – ČL, Czech LargeWhite-sire line – BO and Duroc -D) were used for computation of genetic diversity (GD) and to find the main sources of GD loss. The pedigree completeness indices and coefficients of inbreeding were also calculated for reference population.The average inbreeding varied from 1.4 % to 3.6 % for ČL and D. The number of inbred animals were 91.1 %, 90.7 %, 93.3 % a 69.3 % for ČL, ČBU, BO andD. The total relative GD loss within last generation interval were 7.1 %, 4.7 %, 9.8 % a 7.4 % for ČL, ČBU, BO a D. Ther elative proportion of GD loss due to random genetic drift on total DG loss were8 5 %; 85 %; 90 % and 86 % for ČL, ČBU, BO a D.