Nineteen boys, mean age 14.4 years (range 12.9-16.3), with constitutional delay of growth and puberty were randomised into two groups in a double blind fashion for a three month period. Ten boys received oxandrolone, 2.5 mg per day (mean dose 0.072 mg/kg/day), and nine boys were treated with placebo. Mean growth velocity increased from 4.5 cm/year in the oxandrolone treated group to 9.6 cm/year in three months, and this was sustained at 8.6 cm/year after cessation of treatment. In the placebo treated group, growth rate showed no alteration from 5.1 cm/year to 5.2 cm/year; boys in this group were then treated with oxandrolone, 2.5 mg a day (mean dose 0.073 mg/kg/day) for three months and growth velocity accelerated to 8.6 cm/year. Serum concentrations of insulin-like growth factor -1/somatomedin-C (IGF-1) increased during oxandrolone treatment and continued to rise after treatment had ceased. There was no change in serum IGF-1 concentration during treatment with placebo. Oxandrolone, when used in an appropriate regimen, is an effective, safe treatment for boys with constitutional delay of growth and puberty.