Boys with constitutional delay of growth and puberty (CDGP) should be offered evidence-based effective and safe treatment option. This study compares the effects of low-dose testosterone and aromatase inhibitor letrozole on pubertal progression. The hypothesis is that, in boys CDGP showing earliest signs of puberty, peroral letrozole (2.5 mg/d for 6 mo) induces faster biochemical and clinical progression of puberty as compared to low-dose intramuscular testosterone Rx (~1mg/kg/mo for 6 mo). In addition, 10 or more boys who select watchful waiting instead of medication will provide background data on the natural progression of CDGP, and their data will not be used in primary statistical comparisons.

Inclusion Criteria:

• Constitutional delay of growth and puberty
• Age 14 years or more
• mean testicular volume 2.5 ml or more and less than 4 ml
• serum testosterone level less than 5 nM OR

as above, but serum testosterone 1 nM or more with normal DHEAS level, even if the mean testicular volume is less than 2.5 ml OR

as above, but tanner stage G2 and testosterone level less than 3 nM

Exclusion Criteria:

• Chronic diseases
• Primary or secondary hypogonadism
• Chromosomal anomalies
• Chronic medication that potentially adversely affects bone mineralization (excluding inhaled corticosteroid treatment)