The usefulness of the complete blood count (CBC) during the first week of life in infants with Down syndrome (DS) has been recognized; however, studies are limited and have evaluated only some of the parameters of the CBC. Here, we report a prospective study of 135 infants with cytogenetically confirmed DS and a reference group of 226 infants without birth defects all born during the period 2009-2015 at the Dr. Juan I. Menchaca Civil Hospital of Guadalajara (Guadalajara, Mexico). The goal was to evaluate hematological findings in the CBC during the first 7 days of life, interpreted according to gestational and postnatal age. Data were analyzed using multivariate logistic regression analysis expressed as adjusted odds ratio (aORs) with 95% confidence intervals (95% CIs). Infants with DS had a significantly higher risk for polycythemia (aOR = 12.4, 95% CI: 4.6-33.3), macrocytosis (aOR = 15.9, 95% CI: 1.8-143.4), high values of mean corpuscular hemoglobin (aOR = 36.4, 95% CI: 4.5-294.9), anisocytosis (red blood cells of unequal size) (aOR = 3.9, 95% CI: 2.1-7.6), thrombocytopenia (aOR = 32.4, 95% CI: 15.2-68.9), white blood cell (WBC) count ≥30 × 10 /µl (aOR = 19.4, 95% CI: 4.1-91.5), lymphocytosis (aOR = 73.3, 95% CI: 9.5-565.4), and basophilia (aOR = 16.8, 95% CI: 1.9-151.5). Overall, 74% of infants with DS in our study had polycythemia, thrombocytopenia, WBC count >30 × 10 /µl, or lymphocytosis (aOR = 35.6, 95% CI: 18.8-79.2). Compared with those in other studies, our infants with DS had distinctive hematological findings including a lower frequency of thrombocytopenia, infrequent neutrophilia, and frequent lymphocytosis and neutropenia. This suggests ethnic, socioeconomic, or nutritional differences. © 2017 Wiley Periodicals, Inc.