One-hundred twenty-nine high-risk infants with thrombocytopenia and 238 control infants without thrombocytopenia were evaluated. Mothers of thrombocytopenic babies had similar history to those of nonthrombocytopenic babies, although fewer chronic narcotic abusers were found among mothers of thrombocytopenic babies. Thrombocytopenia was more common in babies less than 37 weeks' gestation and in sick babies compared to healthy babies. Sixty percent of infants had no recognizable cause of thrombocytopenia. Features associated with thrombocytopenia included umbilical line placement, respiratory assistance, hyperbilirubinemia, phototherapy, prematurity, respiratory distress syndrome, low Apgar score, sepsis, meconium aspiration, and necrotizing entercolitis. Thrombocytopenic babies had Apgar score, sepsis, meconium aspiration, and necrotizing entercolitis. Thrombocytopenic babies had more complications, more hemorrhage, and greater mortality than nonthrombocytopenic babies. Platelet size was increased in two babies with immune thrombocytopenia and in none of the others. This study shows that neonatal thrombocytopenia is often associated with high-risk factors and with increased hemorrhage, morbidity, and mortality. This relationship suggests an important prognostic value to platelet size was increased in two babies with immune thrombocytopenia and in none of the others. This study shows that neonatal thrombocytopenia is often associated with high-risk factors and with increased hemorrhage, morbidity, and mortality. This relationship suggests an important prognostic value to platelet counts, although the extent to which the thrombocytopenia contributed directly to morbidity and mortality is not clear.