The aim of this study was to present the trend of inbreeding and to estimate the effect of inbreeding on the growth ability of lambs in the three most common sire breeds in the Czech Republic: Suffolk (SF; n = 49 345), Charollais (CH; n = 14 189) and Texel (T; n = 10 481). The growth ability of lambs was evaluated between 2000 and 2019 based on the weight at 100 days of age. The average inbreeding coefficient for Suffolk, Charollais and Texel was 0.013, 0.012 and 0.011, respectively. The average number of known generations was 5.260 for Suffolk, 3.355 for Charollais and 3.709 for Texel. To evaluate growth ability, lambs were divided into four groups according to the inbreeding coefficient (F = 0.00, 0.00 < F ≤ 0.062 5, 0.062 5 < F ≤ 0.125, F > 0.125). The results of this study show the negative effect of inbreeding on the growth ability of lambs was significant (P < 0.05). The regression coefficients of the weight for Suffolk, Charollais and Texel lambs for a change of 1% in inbreeding were estimated to be –0.028, –0.053 and –0.048, respectively.