Cortisol (CT) concentrations (in micrograms/dl) were determined by radioimmunoassay in plasma obtained at about 3-hr intervals during a 24-hr sampling span from 42 boys and 13 girls of short stature (2-4 standard deviations below their peer group mean), and from a reference group of 11 boys and 10 girls with standard stature, before any treatment were administered to the former. Subjects were 11.20 +/- 0.37 years of age at the time of study, and were living on a diurnal waking (approximately 07:30 to approximately 22:30), nocturnal resting routine during sampling, consuming the usual hospital diet. Circadian rhythm parameters were computed separately for each group by the single and population-mean cosinor fits of a 24-hr cosine curve. A comparison of circadian parameters indicates a statistically significant difference in acrophase (phi; P = 0.033) between short and standard children, as well as added differences in rhythm-adjusted mean (M; P = 0.011) and phi (P = 0.035) between boys and girls of short stature. These differences, as well as any other added information from relevant marker rhythms, should be taken into account for the time-specification of therapy before treatment starts in children of short stature.