Since the elucidation of the congenital nature of Klinefelter's syndrome, considerable interest has been aroused by this not uncommon, chromosomally predetermined, clinical entity. We have undertaken a retrospective study of a large group of patients with this condition in order to establish their phenotypic and laboratory characteristics. Methods The clinical records of 50 patients with Klinefelter's syndrome seen at the Mayo Clinic from 1956 through 1962 were studied. All of the patients were examined by at least one endocrinologist and more than half were interviewed and examined by one or more of the authors. All of the patients had a chromatin-positive buccal smear. Hypogonadal patients who were chromatin negative, as well as one patient with double chromatin bodies in his buccal smear, were excluded from the study. Results History. —The average age of the patients was 38 years (range, 17 to 66). Eleven patients (22%) were more than 50 years