An 8-year-old child had an increased red blood cell mass but did not have splenomegaly, leukocytosis, or thrombocytosis. This disorder is designated primary erythrocytosis of childhood. This form of benign polycythemia may have additional implications in adults. A similar polycythemic state in adult life may have its inception in childhood. Blood studies of family members were performed, and marrow chromosome studies were obtained from the patient and his father. These studies showed a high degree of polyploidy in both, and suggest further similar studies be performed in patients with this disorder.