The effect of a long-acting testosterone preparation1 on growth, on skeletal maturation and on secondary sex characteristics was studied in 24 boys with sexual immaturity of different causes. In patients without growth hormone deficiency dosages above 100 mg/m2/month lead to a maximum growth velocity during the first 6 months of treatment (catch-up growth). Smaller initial dosages followed by a gradual increase to 100–150 mg/m2/month, lead to an imitation of the normal pubertal growth spurt. The time from the start of treatment to the development of axillary hair is inversely proportional to the mean dosage. In patients with growth hormone (GH) deficiency the response is less marked with respect to growth and to the development of secondary sex characteristics. To exert its full growth-promoting anabolic effect, testosterone apparently needs the presence of GH. To exert its full androgenic effect on the secondary sex characteristics, pituitary hormones are also necessary, but it is not clear whether this effect depends on GH or ACTH or on both.