Thrombocytopenia is common in sick neonates, and affected neonates have adverse outcomes compared with those without thrombocytopenia. As impaired platelet production underlies many neonatal thrombocytopenias, affected neonates are potential candidates for hemopoietic growth factor therapy. Although recombinant human (rh) thrombopoietin remains under therapeutic development, rhIL-11, which stimulates megakaryocytopoiesis and increases platelet counts after chemotherapy, is already licensed for clinical use. However, nothing is known about IL-11 in neonates. We therefore measured plasma IL-11 by ELISA in healthy term neonates, stable preterm neonates with or without thrombocytopenia, and preterm neonates with sepsis or necrotizing enterocolitis (NEC) with or without thrombocytopenia. At birth IL-11 was undetectable (<10 pg/mL) in healthy term neonates (n = 20) and 27 of 31 (87%) stable preterm neonates. Three stable preterm neonates with detectable IL-11 (mean+/-SD, 11.3 +/- 0.4 pg/mL; median, 11.6 pg/mL) suffered chorioamnionitis, the remaining neonate (IL-11, 14 pg/mL) being one of nine with early onset thrombocytopenia (present by <72 h of age). IL-11 was also measured in 58 preterm neonates with suspected sepsis or NEC. In 25 of 58, sepsis or NEC was unconfirmed and IL-11 was undetectable. By contrast, 14 of 33 with proven sepsis or NEC had elevated IL-11 (median, 14.9 pg/mL; range, 11.2-92.2 pg/mL). Of these 33 neonates, 19 developed thrombocytopenia: nine of 19 (47%) had detectable IL-11 and 10 of 19 (53%) did not (p > 0.05). Although its role in platelet production in neonates remains unclear, these data suggest that IL-11 is involved in the endogenous cytokine response to sepsis or NEC in preterm neonates. Further studies of IL-11 in neonates are warranted to assess its role both in platelet production and in mediation of the endogenous inflammatory response.