The following case of Klinefelter's syndrome (seminiferous tubule dysgenesis), confirmed by testicular biopsy and chromosome count, is of interest because of the youth of the patient. Report of Case The patient (Fig. 1) is a 10-month-old Puerto Rican boy who was admitted to the hospital because of recurrent episodes of pneumonia. The parents, both 23 years of age, were in good health. The patient was the product of the mother's third pregnancy, the first having terminated in a spontaneous abortion at 6 months. There was a 2-year-old male sibling. Pregnancy and delivery were normal, no medication having been taken by the mother during pregnancy. There was no maternal exposure to x-ray. The patient's birth weight was 8 lb. 6 oz. (3,800 gm.). The patient was bottle-fed, and he sat up at the age of 9 months. Physical examination on admission revealed a well-developed, well-nourished male child with physical