BACKGROUND Controversy exists concerning diet-induced changes to gut epithelia and immunocytes that occur during weaning. Furthermore, studies on dietary effects on the development of the neonatal immune system, especially gut-associated lymphoid tissue (GALT), are lacking. OBJECTIVE The purpose of this study was to investigate growth and development, intestinal morphology, and GALT immune maturation in sow-reared littermates in comparison with early-weaned piglets fed a casein-based liquid diet. METHODS Piglets were breast fed by the sow or were weaned at 48 h to a casein-based diet (formula) that provided the amount of nutrient requirements recommended by the National Research Council. RESULTS Gross physical appearance and visual inspection of the gastrointestinal tract and other organs at necropsy revealed normal organogenesis in both cohorts. On postnatal day 21, body weight, liver and kidney weight relative to body weight, small intestine length, and weight-to-length ratio were greater in formula-fed piglets as compared with sow-reared piglets (p<0.05). The CD21+ B lymphocyte component of GALT and spleen was reduced in the formula-fed piglets. This was associated with lower circulating IgG and IgM levels in the formula-fed as compared with the breast-fed neonatal piglets (p<0.001). CONCLUSIONS Feeding a casein-based formula to newborn piglets may compromise the development of GALT and systemic immune system. Further, the neonatal piglet model may be used to identify the effects of dietary factors on the development of the neonatal immune system.