Mutations in the GHRH receptor (GHRHR) gene (GHRHR) are emerging as a common cause of familial isolated growth hormone deficiency (IGHD) type IB. The use of gonadotropin-releasing hormone (GnRH) analogues has been advocated as a tool to delay puberty in patients with isolated GH deficiency (IGHD), allowing longer time for the beneficial effect of exogenous human GH (hGH) treatment on growth. We describe two male siblings with IGHD due to a homozygous missense GHRHR mutation who, because they were started on hGH therapy at different ages, presented with different height SDS at the onset of puberty and therefore had different predicted target heights. The shorter brother was treated with GnRH analogue plus hGH for 3 years, whereas the other brother received only hGH. Despite different predicted heights at the onset of puberty, they attained similar final heights. We conclude that in patients with IGHD, GnRH analogue treatment should be considered to delay puberty and obtain a maximal growth response if hGH treatment is started in late childhood and the predicted height at puberty onset is below the genetic target.