Familial isolated growth hormone deficiency type IA results from homozygosity for either a 6.7-kb or a 7.6-kb hGH-1 gene deletion. Genomic DNA was extracted from circulating lymphocytes of 78 subjects with severe isolated growth hormone deficiency (height less than -4.5 SD score) and studied by polymerase chain amplification and by restriction endonuclease analysis looking for gene deletions within the hGH-gene cluster. The individuals analyzed were broadly grouped into three different populations (North-European, n = 32; Mediterranean, n = 22; and Turkish, n = 24). Ten out of 78 patients studied presented with an hGH-1 gene deletion; eight out of these 10 showed a 6.7-kb gene deletion, the remaining two a 7.6-kb hGH-1 gene deletion. Five of the 10 subjects developed anti-hGH antibodies to hGH replacement followed by a stunted growth response. Family studies of the affected patients were performed, revealing consanguinity in all the families, and the corresponding heterozygosity for the deletion was present in each of the parents. The results of our study revealed a prevalence for an hGH-1 gene deletion in three out of 32 North-European, three out of 22 Mediterranean, and four out of 24 Turkish patients with growth hormone deficiency (height less than 4.5 SD score). These data are important for prenatal diagnosis of at-risk pregnancies and for families at risk for recurrence and underline clearly the fact that the hGH-I gene deletion represents a common cause for growth hormone deficiency associated with severe growth retardation (height less than -4.5 SD score).