. 2010. Relationships between backfat thickness and reproductive efficiency of sows: A two-year trial involving two commercial herds fixing backfat thickness at breeding. Can. J. Anim. Sci. 90: 429Á436. In this study, we established whether controlling backfat thickness at breeding over a long time period can result in optimized reproductive performance in sows. Two commercial herds were used: herd A (322 purebred Landrace sows) and herd B (337 cross-bred Yorkshire-Landrace sows). Backfat thickness at breeding and farrowing, along with reproductive data [live Born (LB), stillborn, mummified, piglets alive at 48 h (LB48) and the weaning-to-estrus interval (WEI)] were collected over nine parities. The herd B producer was more successful in maintaining a steady backfat thickness at breeding than was the herd A producer. At breeding, the backfat thickness of sows from herd A showed a marked decrease between parities 2 and 5. During their first parity, these sows gained the least backfat during gestation and lost the most backfat during lactation. Sows from herd B had more LB and LB48 than sows from herd A. In herd A, a longer WEI was found in first and second parity sows. Our results demonstrate that maintaining backfat thickness throughout the reproductive cycle is more important than fixing this parameter at breeding alone. This is particularly true for gilts, which are prone to mobilize fat tissue reserves, a condition associated with declining reproductive performance.