Here we present two siblings, a 9-year-old boy and a 15-year-old girl at presentation, with congenital erythrocytosis due to an EPOR c.1316G>A (p.Trp439Term) mutation. The patients had nausea, abdominal pain, and headache when they presented with hemoglobin levels of 23 g/dL and 19.4 g/dL, respectively. Their father, paternal uncle, and probably the paternal aunt and grandmother had congenital erythrocytosis. The siblings generally preferred to visit hospital when hyperviscosity symptoms developed and had intermittent phlebotomies. Their compliance to anti-aggregant and hematinic treatment was not satisfactory. Within the 11-year follow-up period, the siblings had no thrombohemorrhagic complications, whereas their 39-year-old uncle had a stroke. In addition to antiaggregant treatment, phlebotomy during hyperviscosity symptoms may be safe in children and adolescents; routine phlebotomies may be recommended to adults to prevent thrombohemorrhagic complications.