The Lesch-Nyhan syndrome is an inherited neurologic disorder due to deficiency of hypoxanthine-guanine phosphoribosyl transferase and characterized clinically by self-mutilation, choreoathetosis, spasticity and mental retardation. This study demonstrates that these patients have significant growth retardation similar to that observed in other groups of patients with mental retardation. Endocrine studies indicate that this growth retardation cannot be attributed to pituitary or thyroid dysfunction. Serum somatomedin (sulphation factor) measurements were within the range of normal in 4 of the 5 patients studied. Therefore, the defect in growth and osseous maturation must be due to an abnormality in the end organ (cartilage).