Background: The immune modulator TNF-related apoptosis-inducing ligand (TRAIL) has been found at extremely high levels in human milk of women with normal gestation at day 5 after delivery. Objective: To investigate the presence and the levels of soluble TRAIL in human milk of women with preterm delivery at different time points post-partum (32, 34, and 36 weeks from conception). Methods: The levels of soluble TRAIL were analyzed by ELISA in the breast milk of a group of 25 women with preterm delivery at different gestational ages. Results: Soluble TRAIL was present at high levels in human milk since early post-conceptional ages (32 weeks). No significant differences in TRAIL levels were noticed with respect to different gestational ages, or with respect to time of collection when comparing, in a selected group of patients, samples obtained between 15 and 26 days with those obtained 27 and 40 days after birth. Conclusion: Due to the key immunoregulatory role of human soluble TRAIL, the presence of high levels of TRAIL in the milk of women with preterm delivery and its maintenance at high levels up to 72 days after birth support the importance of breastfeeding the preterm newborn.