L-Leucine is a signaling amino acid in animal metabolism. It is unknown whether supplementing L-leucine to breast-fed neonates may enhance their small-intestinal development. This hypothesis was tested with a piglet model. Seven-day-old sow-reared pigs with an average birth weight of 1.45 kg were assigned randomly to the control or leucine group (n = 30/group). Piglets in the leucine group were orally administrated with 1.4 g L-leucine/kg body weight, whereas piglets in the control group received isonitrogenous L-alanine, twice daily for 14 days. The supplemental L-leucine amounted to 200 % of L-leucine intake from sow's milk by 7-day-old pigs. At the end of the 2-week experiment, tissue samples were collected for determining intestinal morphology, expression of genes for intestinal leucine transporters (real-time RT-PCR and western blot analysis), and plasma metabolites and hormones. L-leucine administration increased (P < 0.05) villus height in the duodenum, an elevated ratio of villus height to crypt depth in the duodenum and ileum, plasma concentrations of leucine, glutamine and asparagine, as well as body-weight gains. mRNA levels for L-leucine transporters (SLC6A14, SLC6A19 and SLC7A9) and the abundance of the ATB(0,+) protein were increased (P < 0.05) but those for SLC7A7 mRNA and the LAT2 protein were decreased (P < 0.05) in the jejunum of leucine-supplemented piglets, compared with the control. Plasma concentrations of ammonia, urea, triglycerides, and growth-related hormones did not differ between the control and leucine groups. Collectively, these results indicate that L-leucine supplementation improves intestinal development and whole-body growth in suckling piglets with a normal birth weight.