We describe here a 19 month-old girl with classical Laron syndrome (LS). Molecular analysis of the GH receptor gene in the patient and her parents was performed. The patient was found to be heterozygous for a mutation in exon 4 (R43X) and heterozygous for a polymorphism in exon 6 (Gly168Gly). Her mother was also heterozygous for R43X but homozygous for the polymorphism. In the father, a heterozygous polymorphism was found. Contrary to previous assumptions that only homozygous patients express the typical phenotype, this patient shows all the classical features of LS, despite being a heterozygote for a pathological defect.