Hematologic findings and published complete blood cell count criteria were evaluated as screening tests for neonatal sepsis. From the data obtained, a hematologic scoring system was formulated that assigns a score of 1 for each of seven findings: abnormal total leukocyte count, abnormal total neutrophil (PMN) count, elevated immature PMN count, elevated immature to total PMN ratio, immature to mature PMN ratio greater than or equal to 0.3, platelet count less than or equal to 150,000/mm3, and pronounced degenerative changes in PMNs. There were 298 evaluations for sepsis (243 in the first 24 hours of life and 55 between days 2 and 30). Twenty-six of 27 (96%) infants with sepsis and all 23 infants with probable infection had scores greater than or equal to 3, compared with 35 of 248 (14%) noninfected infants. The likelihood of sepsis with score greater than or equal to 3 was 31%, and this value differed with both gestational and postnatal ages (34% vs 8% in preterm and term infants less than 24 hours of age, and 65% thereafter). The higher the score the greater was the likelihood of sepsis. With score less than or equal to 2 the likelihood that sepsis was absent was 99%. The hematologic scoring system should improve the diagnostic accuracy of the complete blood cell count as a screening test for sepsis and could simplify and standardize the interpretation of this global test.