A detailed study involving simultaneous measurements of plasma and saliva 17OH-progesterone (17OHP), and plasma testosterone concentrations was performed at frequent intervals over a 3-year period in 16 patients with congenital adrenal hyperplasia (CAH). There was a close correlation between the results of these three biochemical measurements over a wide range of concentrations. The practical application of a sensitive saliva 17OHP radioimmunoassay permitted detailed monitoring of patients receiving various glucocorticoid preparations through repeated frequent saliva sampling over the whole day. When the results of serial steroid measurements were analysed in relation to growth velocity in prepubertal patients, it was possible to devise upper limits of 40 nmol/l, 0.8 nmol/l and 1,500 pmol/l for plasma 17OHP, plasma testosterone and saliva 17OHP concentrations, respectively, in well-controlled patients. Applying these guidelines from the early onset of treatment should ensure normal growth potential in treated CAH children, at least until puberty.