A sensitive immunochemiluminometric assay with a detection limit of 1.1 microU/L was developed for the measurement of urinary growth hormone (UGH). The assay was shown to be specific and precise. There was a good correlation between serum growth hormone (GH) and UGH concentrations in 20 patients with acromegaly and six volunteers following an intravenous injection of recombinant GH. We concluded therefore that UGH measurements appear to provide a satisfactory index of GH secretion. The use of the assay in the investigation of growth disorders was assessed. We studied 11 pre-pubertal children, six of normal stature, and five of short stature, over a 6-month period. Sequential fortnightly measurements of UGH were carried out and height velocity was determined. The children of short stature grew at a slower rate and excreted less GH than the children of normal stature. However, we observed considerable within-individual variability in GH excretion in both groups (CV 22-98%). We therefore recommend that sequential UGH analyses should be carried out and the results interpreted in conjunction with growth measurements. However, further investigations into the renal handling of GH are needed to establish optimum sampling regimes.