Purpose: Lesch–Nyhan disease is an inherited metabolic disorder characterized by overproduction of uric acid and neurobehavioral abnormalities. The purpose of this study was to describe macrocytic erythrocytes as another common aspect of the phenotype. Methods: The results of 257 complete blood counts from 65 patients over a 23-year period were collected from 2 reference centers where many patients are seen regularly. Results: Macrocytic erythrocytes occurred in 81–92% of subjects with Lesch–Nyhan disease or its neurological variants. After excluding cases with iron deficiency because it might pseudonormalize erythrocyte volumes, macrocytosis occurred in 97% of subjects. Macrocytic erythrocytes were sometimes accompanied by mild anemia, and rarely by severe anemia. Conclusion: These results establish macrocytic erythrocytes as a very common aspect of the clinical phenotype of Lesch–Nyhan disease and its neurological variants. Macrocytosis is so characteristic that its absence should prompt suspicion of a secondary process, such as iron deficiency. Because macrocytosis is uncommon in unaffected children, it can also be used as a clue for early diagnosis in children with neurodevelopmental delay. Better recognition of this characteristic feature of the disorder will also help to prevent unnecessary diagnostic testing and unnecessary attempts to treat it with folate or B12 supplements.