When studying short slowly growing children, several investigators have found differences in spontaneous circulating growth hormone (GH) levels in some short children as compared with children of normal stature. The circadian and ultradian characteristics of GH secretion have not been considered in most of those studies. With the aim to study possible differences in rhythm characteristics of plasma GH with stature, we analyzed data from a total of 141 prepubertal children: 1) 13 GH-deficient children; 2) 36 children with short stature (up to 2 standard deviations below their peer group mean); 3) 61 children with very short stature (up to 4 standard deviations below their peer group mean); and 4) a reference group of 31 children with standard stature. Subjects were living at the hospital setting on a diurnal waking (07:30 a.m. to 10:30 p.m.), nocturnal resting routine during sampling, consuming the usual hospital diet at fixed times. GH concentrations were determined by immunoradiometric assay in plasma obtained at about 2-3 hour intervals during most of the day and at about half-hour intervals between 9:00 p.m. and 3:00 a.m. Circadian rhythm characteristics obtained by least-squares estimation were compared between groups divided according to gender and stature with a parameter test. Results show a statistically significant circadian rhythm in GH secretion for all groups studied (p<0.001 for standard, short and very short children; p=0.013 for GH-deficient children). A comparison of circadian parameters indicates similar characteristics between subjects of short, very short and standard stature. The difference in rhythm-adjusted mean and amplitude is, however, of borderline statistical [correction of statistically] significance when comparing non-deficient with GH-deficient children.