We sought to analyze associations between aberrant hematologic counts and adverse outcome in neonates with hypoxic ischemic encephalopathy (HIE) due to various intrapartum asphyxial insult types and to describe the postnatal changes in counts. Hematologic counts between 0 and 120 hours of age of 316 consecutively admitted neonates with HIE were collected retrospectively. Asphyxial insult types were categorized as acute near-total, prolonged partial, or "mixed." Associations between hematologic counts and adverse outcome by 2 years of age were analyzed. No associations were found between patterns of hematologic counts in the first 12 hours and adverse outcome. Lymphocyte counts peaked in the first 2 hours of age in all insult types (46% had values > 10.0 x 10 (9)/L) and by 4 to 6 hours reached normal levels. Nucleated red blood cell counts peaked between 6 and 8 hours of age and fell to normal levels by 36 to 72 hours of age (56% had values > 1860 x 10 (6)/L). Relatively few subjects had low platelet counts by age 12 hours; the nadir occurred on days 2 to 3. No associations were found between hematologic counts and adverse outcome. The characteristic hematologic changes found are attributable to the asphyxial insults, not to brain injury. Because of inconsistent changes, hematologic counts cannot be used on their own to time asphyxial insults.