Limited information is available on the expected hematological values of newborn infants with trisomy 18 or trisomy 13 syndromes. About 50% of patients with these conditions survive beyond the first week of life, and some have complete blood counts (CBCs) obtained to assist in medical management. We tabulated all CBC data that had been obtained on patients with either trisomy, during their first week of life, using data from an 18 hospital healthcare system. We posited that describing the CBC findings would assist clinicians in providing care for these patients who survive beyond the first days. During the period of January 1, 2001 through December 31, 2006, 174,480 live births were recorded at the 18 hospitals. Trisomy 18 was recognized in 28 (prevalence estimate, 1 in 6,231) and trisomy 13 in 22 (prevalence estimate, 1 in 7,931). Twenty-four of these had one or more CBCs obtained before the seventh day. Thrombocytopenia was the most common hematological abnormality detected, occurring in 83% of those with trisomy 18 and 75% of those with trisomy 13. Three patients with trisomy 18, and 1 with trisomy 13, received platelet transfusions. The second most commonly detected abnormality was neutrophilia. Eighty-three percent of neonates with trisomy 13, and 42% with trisomy 18, had neutrophil concentrations above the upper limit of normal for age. Abnormal erythrocyte values were the third most common hematological abnormalities detected. These were much more common among neonates with trisomy 18. Only 43% of patients with trisomy 18 had normal erythrocyte values; anemia was detected in 40% and polycythemia in 17%. Only one neonate with trisomy 13 had abnormal erythrocyte findings (polycythemia). These data should assist clinicians caring for neonates with trisomy 18 or 13, demonstrating how common hematological abnormalities exist among these patients.