The current literature indicates that several abnormalities have been observed in the three hematopoietic cell lines of infants with Down's syndrome. This prospective, longitudinal study was designed to clarify the physiological variation in peripheral blood cell values of children with Down's syndrome by following 25 such infants during their first year of life. Apart from polycythemia in the first week of life, the hemoglobin concentration was, in general, the same as in normal term infants. At 9-12 months of age values for mean corpuscular hemoglobin and mean corpuscular volume tended to be elevated. Serum erythropoietin concentrations were low to normal. White blood cell counts were slightly lower in children with Down's syndrome than in normal children. The study infants had profound thrombocytosis from the age of 6 weeks to the end of follow-up at 1 year. This study, the first longitudinal follow-up of such subjects, indicates that infants with Down's syndrome often have evidence of polycythemia soon after birth and red blood cell macrocytosis and thrombocytosis later in infancy. In conclusion, we carried out peripheral blood cell counts in 25 infants with Down's syndrome, but with no actual hematological disturbance, during their first year of life, and compared them with values for normal term infants.