Three siblings with Laron syndrome (LS) and normal serum growth hormone binding protein (GHBP) are described. Basal serum levels of hGH were high and IGF-1 low, and in contradistinction to the classical form of the disease serum GHBP and IGFBP-3 were normal in these patients. After 7 days of human growth hormone administration serum IGFBP-3 levels as well as the number of red blood cell IGF receptor sites increased. After short- and long-term IGF-1 administration the IGF-1 receptor binding capacity as well as the number of IGF receptor sites decreased to levels found in control subjects. One year treatment with IGF-1 increased the growth velocity by 47-96% in the two older children. It is concluded that the findings described are compatible with a normal GH receptor and normal signal transmission for IGFBP-3 synthesis but a defect exists in the post-GH receptor mechanism for the generation of IGF-1. This is the first description of this type of defect leading to a variant of Laron syndrome.