In patients with growth hormone (GH) deficiency (GHD), low doses of recombinant human GH (rhGH) have a similar or better long-term clinical effect than higher doses. Pharmacogenetic studies suggest that GH receptor (GHR) polymorphism only influences some metabolic parameters. Nonetheless, there is no clear scientific evidence proving the effects of lower rhGH dose regimens on metabolic parameters. The aim of our prospective study was to evaluate the effects of GHR polymorphism in adult GHD patients treated with low rhGH dose during short- (6 and 12 months) and long-term (5 years) follow-up. Sixty-nine GHD adult patients were studied, before and during treatment with rhGH, using a standardised low-dose protocol calculated on the basis of body weight (0.01-0.03 mg kg-1 week-1 ) and monitored by an insulin-like growth factor (IGF)-I plasma assay, as well as anthropometric and metabolic parameters. The GHR genotype (flfl, fld3 or d3d3) was determined from the peripheral blood. d3-GHR carriers showed a more effective short- and long-term response to low rhGH dose with respect to low-density lipoprotein reduction, body composition and blood pressure (homozygous patients only); d3-GHR homozygosity is related to a significant IGF-I increase during short-term follow-up. Regression analysis demonstrated that rhGH dose, age at diagnosis and GHR genotype are the major determinants of IGF-I increase at 6 and 12 months of replacement therapy. The d3d3-GHR genotype may influence some metabolic effects during the short- and long-term follow-up of low rhGH dose and could be an independent determinant of the increase of IGF- I during short-term follow-up.