The bone mineral content (BMC) and body height were measured in 301 normal children and adolescents aged 7--20 years, and in 8 boys with constitutional delayed puberty aged 14--17 years. Serum testosterone was measured in the last group as well as in a subpopulation of the normal children and adolescents. The growth spurt, which coincided with a steep increase of serum testosterone in boys, indicated a great change in skeletal growth and mineralisation in both sexes. After the growth spurt, linear growth slowed down considerably while bone mineralisation rose steeply. When low levels of serum testosterone were maintained, as in delayed puberty, these combined changes of skeletal growth and mineralisation did not occur. It is suggested that gonadal hormones are the true initiators of the short-lived growth spurt as well as of prolonged acceleration of bone mineralisation.