Leptin (LEP), the product of the leptin gene, is a 16-kDa protein synthesised by adipose tissue and is involved in regulation of feed intake, energy balance, fertility, and immune functions. Using bi-directional polymerase chain reaction amplification of specific alleles (bi-PASA) and PCR-restriction fragment length polymorphisms (PCR-RFLP), we have successfully detected genetic polymorphisms of LEP intron 1 and exon 3, respectively, in Landrace, Yorkshire, and Duroc. Chi-square statistics were used to calculate homogeneity of genotypic frequencies of 2 candidate genes for 3 breeds of animals. Effects of exon 3 polymorphisms on reproduction traits such as litter sizes of sows in the first parity and 1–4 parities were evident (P < 0.05) in Landrace, and 1–4 parities in Yorkshire. Polymorphisms of intron 1 affected (P < 0.05) only litter sizes of the first parity in Duroc. Effects of exon 3 polymorphisms on production traits such as average daily gain and feed efficiency were evident (P < 0.05) in Landrace and Yorkshire, respectively. Polymorphisms of intron 1 affected (P < 0.05) only backfat thickness in Duroc. Our data also show that the higher the reproduction traits, the lower the genotypic frequencies. Further, the better the production traits, the higher the genotypic frequencies were. These data suggest that the long-term selection of favourable traits in pig production may be partially attributed to the effects of LEP polymorphisms on reproduction and production traits.