SUMMARY The technique of cortisol estimation by competitive protein binding analysis has been applied to the measurement of urinary free cortisol excretion. It has been found to be very sensitive and highly reproducible. There is good agreement with estimates of free cortisol excretion obtained by a chromatographic technique. The mean normal excretion by a group of 24 convalescent patients without endocrine disease was 41·9 μg./24 hr. The range was 0 to 145·5 μg./24 hr. Eight obese patients showed values well within the normal range. The values found in eight cases of Cushing's syndrome ranged from 98·6 to 6100 μg./24 hr. There was very little overlap with the normal range. Urinary free cortisol excretion was found to be a sensitive index of adrenal function during corticotrophin stimulation or dexamethasone suppression.