In the absence of any gastroenterologic complaint, circadian variations characterize serum GL in 22 boys and 7 girls of short stature, 6-14 years of age, living on a diurnal waking (approximately 07:00 to approximately 22:00), nocturnal resting routine, consuming the usual hospital diet. Blood was drawn at about 3-hour intervals during a 24-hour sampling span and serum stored frozen at -60 degrees C until radioimmunoassay for GL concentration (in mg/dl). Circadian rhythm parameters were computed by the single and population mean cosinor fits of a 24-hour cosine curve. A parameter-comparison of data from the two genders indicates similar circadian M, A and 0 for both groups. The Ps for a comparison of Ms, As, 0s and (A,0) pairs are .635, .183, .385 and .308, respectively. The larger differences between genders were found for the circadian A. In human beings, differences in the A of a rhythm are associated with differences in the probable development of several civilization diseases. In this sense, evaluation of risk by appropriately designed questionnaires should next complement blood sampling in groups of children differing in age, gender, health or ethnicity, with the purpose in mind of deriving prediction regions for assessment of health, risk or disease.