A brief course of testosterone injections is known to be an effective treatment for boys with constitutional delayed puberty. In this study, data from seven boys at least 14 years old who received testosterone enanthate (100 mg intramuscularly monthly for four months) were analyzed to see if linear and testicular growth responses could be useful diagnostically in excluding growth hormone deficiency (GHD) and isolated gonadotropin deficiency, two conditions that are often difficult to distinguish from constitutional delayed puberty. During four months of testosterone therapy, growth rate increased from 4.0 +/- 1.0 cm/y to 10.7 +/- 2.3 cm/y, and was greater than 8 cm/y in all patients. Since testosterone-induced stimulation of linear growth is largely GH-mediated, the large increase in growth rate in all boys is considered indicative of GH sufficiency. Testis length, which did not increase during testosterone therapy, increased by 0.6 to 0.8 cm in every patient (from 2.7 +/- 0.3 cm to 3.4 +/- 0.4 cm) over the following four months, indicating normal gonadotropin secretion and normal pubertal progression; in contrast, the increase in serum testosterone concentrations after discontinuation of testosterone treatment was more variable. It is concluded that the growth response to a four-month course of testosterone is helpful in excluding GHD in boys with delayed puberty, and an additional four months of follow-up is sufficient to document the onset of puberty, thereby eliminating the possibility of isolated gonadotropin deficiency.