Pre-weaning performance of piglets who had been cross-fostered was compared to evaluate the effects of litter size and littermate weight on low-birth-weight piglets. Litter size was either 8 or 12 piglets. Low-birth-weight piglets were housed with either an equal number of heavy-birth-weight piglets, an equal number of average-birth-weight piglets, or in a group of all low-birth-weight piglets. A resulting six groups (treatments) were created. In large litters, the mortality of low-birth-weight piglets housed with heavy-birth-weight piglets was greatest. In small litters, mortality of low-birth-weight piglets housed with heavy-birth-weight piglets was no different. Neither littermate weight, litter size, nor the interaction between the two affected weight gains of the piglets until day three post partum. Conversely, littermate weight, litter size, and the interaction between the two did affect weight at 21 days post partum. Low-birth-weight piglets in small litters had a significantly higher 21 day weights than piglets in large litters. In large but not small litters, low-birth-weight piglets were absent at more nursing episodes and spent more time fighting over teats than their heavier littermates, whether housed with heavy-birth-weight piglets or average-birth-weight piglets. Conclusions stated that littermate weight may affect performance of low-birth-weight piglets due to direct competition for access to a functional teat.