Serial blood spot and saliva samples were collected at home by 18 patients being treated for congenital adrenal hyperplasia to determine the circadian rhythm of 170H-progesterone as an index of therapeutic control. There was a strong correlation between the magnitude of the circadian fall and a single morning measurement of the plasma testosterone concentration taken near the time of the 170H-progesterone rhythm samples. Poor control in pubertal girls produced an exaggerated circadian fall in 170H-progesterone concentrations that were raised at all sampling times. Optimal control (plasma testosterone 1.5-2.5 nmol/l) was associated with blood spot and salivary 170H-progesterone concentrations at 0800 hours of between 30-70 nmol/l and 260-1000 pmol/l, respectively, falling thereafter to less than 10 nmol/l and less than 150 pmol/l, respectively. Similar results were obtained in prepubertal patients. Nomograms have been constructed to interpret the daily profiles of blood spot or salivary measurements of 170H-progesterone, or both. The analysis of 170H-progesterone circadian rhythms is useful in monitoring treatment in patients with congenital adrenal hyperplasia, particularly those who may be overtreated.