Nineteen boys with true isolated growth hormone deficiency developed the first stages of puberty at an average bone age of 12.0"years" (Tanner Whitehouse Method 2, RUS score). The average chronological age was 15.0 years. Seven similar girls entered puberty at 10.9"years" in bone age and 13.7 years in chronological age. The means and ranges of bone age at beginning of puberty of these patients are very close to those of normal children.