• We investigated growth features, development of primary and secondary sex characteristics, and specified pituitary and testicular hormone levels in 46 male adolescents and young adults with Down syndrome. Their mean height age was significantly less than and their mean bone age was slightly more than their chronological age. The subjects' secondary sex characteristics followed the same developmental pattern noted in youngsters without Down syndrome. Penile length and circumference and testicular volume of our patients with Down syndrome were not statistically different from those of normal adolescents. Follicle-stimulating hormone, luteinizing hormone, and testosterone levels in our study population were similar to those reported for normal adolescents during sexual maturation.