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Genotype effects on lamb growth, survival, and commercial finishing were estimated in a 5-yr study aimed at assessing potential benefits from introducing meat breeds into extensive sheep systems of northeastern Patagonia. Five ram [Corriedale: CO; Border Leicester: BL; Ile de France: IF; Texel: TX; and synthetic CRIII (25% Merino, 37.5% IF, 37.5% TX)] and 5 dam (CO; synthetic CRIII; BLCO: BL x CO; IFCO: IF x CO; and TXCO: TX x CO) genotypes were represented in the study. Data were collected from 1,426 born and 1,258 weaned lambs of 9 resulting genotypes (CO x CO, BL x CO, IF x CO, TX x CO, CRIII x CO, CRIII x BLCO, CRIII x IFCO, CRIII x TXCO, and CRIII x CRIII). Birth weight was recorded on all lambs; subsequently, BW and BCS (scale 1 to 5) were recorded at regular intervals until weaning. Body weights were adjusted to 60 and 90 (weaning) d of age, and ADG were calculated from the adjusted BW for the periods 0 to 60 d and 60 to 90 d; BCS was adjusted to 90 d. Survival to weaning and percentage of lambs reaching commercial finishing (BW >or= 23 kg and BCS >or= 2.5 points) were also recorded. Significant (P < 0.05) genotype x litter size interactions were detected for birth weight and ADG 60 to 90 d. With the exception of CRIII x CO, crossbred and synthetic genotypes presented greater (P < 0.05) ADG 0 to 60 d and BW at 60 and 90 d than CO x CO lambs. Second cross lambs reared as singles presented greater ADG 60 to 90 d (P < 0.05) than BL x CO, TX x CO, and CRIII x CO, but less (P < 0.05) ADG 60 to 90 d, and no differences were observed for twins. The IF x CO, CRIII x BLCO, CRIII x IFCO, and CRIII x CRIII genotypes showed greater BCS at 90 d (P < 0.05) than CO x CO. The probability of greater commercial finishing for crossbred and synthetic genotypes relative to CO x CO was at least 79%. Probabilities of greater survival to weaning in CO x CO and CRIII x BLCO lambs relative to IF x CO, TX x CO, CRIII x CO, and CRIII x CRIII lambs were greater than 81%. Results indicate significant improvements in lamb BW and saleable lambs from the introduction of meat genotypes. Among the terminal sire breeds evaluated, BL and IF would produce the greatest impact. Differential nutritional management of pregnant ewes carrying twins and of twin lambs beyond 60 d should be implemented to mitigate litter size x genotype interactions constraining growth potential benefits, which may be critical for northeastern Patagonia conditions.