OBJECTIVES: Reticulated platelets (RPs) are newly synthesized platelets with increased ribonucleic acid content. The percentage of RPs is elevated in adults with thrombocytopenia as a result of increased platelet destruction. The objectives of this study were to determine normal RP values in neonates at birth and to determine whether neonates with thrombocytopenia as a result of increased platelet destruction have an increased percentage of RPs. STUDY DESIGN: The RP percentages were measured at birth in 89 neonates without thrombocytopenia in three gestational age groups (<30, 30 to 36, and >36 weeks), six neonates with immune thrombocytopenia, and one neonate with thrombocytopenia as a result of decreased platelet production. RESULTS: The RP percentages in neonates without thrombocytopenia >36 weeks and 30 to 36 weeks of gestation were 4.0% ± 2.4% (mean ± SD) and 4.6% ± 1.7%, respectively, similar to values reported in healthy adults. Neonates younger than 30 weeks of gestation had significantly higher RP percentages (8.8% ± 5.1%) than older neonates (p ≤0.0001). Term neonates with immune thrombocytopenia had significantly higher RP percentages (38.3% ± 23.1%) than term neonates without thrombocytopenia (p = 0.0001), with no overlap in RP values. A lower RP percentage (1.5%) and significantly lower absolute RP count were measured in the one neonate with decreased platelet production. CONCLUSIONS: These preliminary data establish normal RP percentages in term and preterm neonates at birth and demonstrate elevated RP percentages in neonates with immune platelet destruction. The RP values may be helpful in evaluating the causes of neonatal thrombocytopenia. (J PEDIATR 1996;129:269-74)