The relationship between 18-month mating mass and average lifetime conceptions/mating (Ec/Em), lambs born/conception (Lb/Ec), lambs born/mating(Lb/Em), preweaning mortalities/Iamb born (Ld/Lb) and lambs weaned/mating (Lw/Em) was investigated with data from 1222 Merino ewes available for at least four lambing opportunities in the Tygerhoek flock. The regressions of average lifetime Ec/Em, Lb/Ec, Lb/Em and Lw/Em on 18-month mating mass were linear, positive and statistically significant (P ≤0,001). Pre-weaning mortalities were independent of 18-month mating mass. An increase of 1 kg in 18-month mating mass was associated with respective increases of 0,007 Ec/Em, 0,018 Lb/Ec, 0,024 Lb/Em and 0,017 Lw/Em. These results were supported by subsequent analyses, where average lifetime Ec/Em, Lb/Em and Lw/Em were used as discrete independent variables, and 18-month mating mass as the dependent variable. Phenotypic correlations obtained between 18-month mating mass and average lifetime Ec/Em, Lb/Ec, Lb/Em and Lw/Em were 0,11, 0,25,0,24 and 0,18 respectively.