Serum levels of somatomedin-C have been measured by a competitive membrane binding assay in normal individuals, in hypopituitary dwarfs, and in children with growth retardation from causes other than growth hormone deficiency. The mean level in untreated hypopituitary children is about 40% of that of age-matched normal children. Treatment of hypopituitary dwarfs with human growth hormone results in prompt increases in the serum concentration of somatomedin-C. Normalization of somatomedin is associated with the return of pituitary function in hospitalized psychosocial dwarfs. Supporting the possibility that somatomedin-C plays a pivotal role in skeletal growth is the finding that serum levels in hypopituitary children correlate with growth rate both on and off growth hormone therapy. In our hands, this assay has proved to be an efficient, reliable method for the diagnosis of growth hormone deficiency.