The possible influence of the pig major histocompatibility complex on reproductive traits is reviewed. Among the parameters investigated, significant associations were observed between SLA and genital tract development in males. Several SLA haplotypes affected either positively or negatively the development of testes, the epididymes and the Cowper's glands, whereas the influence of these haplotypes on androsterone tissue content was limited. The impact of the SLA complex on female reproductive performance has been more difficult to ascertain, although some results suggest that the SLA region may interfere with ovulation rates. The influence on prolificacy of SLA sharing between sires and dams was also investigated. Even if fertilization rate is not affected, embryonic mortality and preimplantation embryo development might be influenced by the pig MHC region. Although the litter size was not only marginally affected by the SLA complex, there is one case where SLA identity between the boar and several related sows led to significantly reduced litter size. In these families piglets homozygous for a particular SLA haplotype were almost absent, suggesting the existence of a recessive lethal gene linked to SLA.