The description by Barr and Bertram1of a sex-related morphologic difference in the nuclei of neuronal cells of the cat central nervous system has led to the development of a new diagnostic tool in the study of certain pediatric endocrine disorders. A nuclear morphologic difference between the sexes has been reported for the cat, dog, mink, marten, ferret, skunk, raccoon, goat, deer, monkey,2,3and man.2-6The test as originally devised for application to human subjects utilized a skin biopsy specimen.5,7,8It has been modified to use cells from such actively growing areas as the oral mucosa,6,9,10vaginal mucosa,11and amniotic membranes.12,13The chromosomal sex test has been applied to patients with gonadal dysgenesis,6,8,9,14,15female pseudohermaphroditism,3,5,7,6,15male pseudohermaphroditism,14,18true hermaphroditism,15,18and Klinefelter's syndrome.19-21 The present report illustrates the usefulness and limitations of this technique in clinical pediatrics. Materials