Background: Although congenital erythrocytosis (CE), an inherited disorder, impairs pediatric quality of life, physicians often overlook high hemoglobin (Hgb) levels and its symptoms due to lack of knowledge of age-adjusted pediatric Hgb levels and CE’s rarity. Methods: In a retrospective, single-center study, data from hospital records of patients (<18 years) diagnosed with CE were evaluated. Results: Forty patients from 39 families (male: female ratio = 7:1) had been diagnosed with CE in a 20-year period, at a mean age of 15.31 ± 2.49 years (8.34–17.92) and with mean Hgb levels of 17.4 ± 1.34 g/dL (14.63–23.0). No serum erythropoietin levels exceeded the reference levels. Although the most common symptom was headache (80.0%), 40.0% of patients presented with at least one gastrointestinal symptom (e.g., nausea, vomiting, abdominal pain, and rectal bleeding), and 75.0% exhibited plethora. None had leukocytosis, thrombocytosis, abnormal capillary oxygen saturation, JAK2 mutation, and venous blood gas analysis and Hgb electrophoresis revealed no abnormalities. While 43.6% of patients had family histories of CE, 28.2% had 15–48-year-old relatives who had experienced myocardial infarction, stroke, and/or sudden death. Six asymptomatic patients were detected incidentally. When symptoms of hyperviscosity were present, aspirin was prescribed, and phlebotomy performed. No thrombotic episodes occurred as a result. Conclusion: To detect CE, physicians should assess Hgb levels in consideration of normal age-adjusted levels in children. Pediatric patients with CE may also present with gastrointestinal symptoms. Although no thrombotic episode occurred among the patients, their family histories included life-threatening thrombotic episodes, even in adolescents.