Expression of heterozygosity for the defect in the growth hormone (GH) receptor has been proposed to be reflected in stature, and in GH binding protein (GHBP) and insulin-like growth factor I (IGF-I) levels in parents and other relatives of patients with GH receptor deficiency (GHRD; Laron syndrome). The Ecuadorean population with GHRD, in which heterozygosity can be accurately determined in clinically unaffected relatives of probands, offers a unique opportunity to consider this issue. It has previously been demonstrated that 17 parents heterozygous for the Ecuadorean mutation of the GH receptor differed little in biochemical measures (GHBP, IGF-I, IGF-II, IGFBP-2 and IGFBP-3) from Ecuadorean controls. Mean height SDS of 24 non-carrier siblings (-1.3 +/- 0.95 SD) and 41 heterozygote siblings or offspring of probands (-1.8 +/- 1.15) did not differ significantly (p = 0.08). Thus, although there may be slight heterozygote expression of the defective gene for the GH receptor, there is no rationale for counselling based on such minimal variation.