Thirty-seven prepubertal children evaluated for severe growth retardation were studied by assessment of total granulocyte, monocyte and lymphocyte count, lymphocyte subsets CD3+, CD3+Dr+, CD3+Dr-, CD4+, CD8+, CD8+CD57+, CD8+CD57-, CD16+, CD20+ and CD23+, serum immunoglobulin concentrations, and phagocytic activity of circulating neutrophils and monocytes (by a flow cytometric assay). Idiopathic GH deficiency was diagnosed in 21 of 37 patients; the remaining 16 healthy subjects served as controls. Fourteen patients received biosynthetic GH (rhGH), and their immune parameters were assessed at baseline and after 6 months of therapy. Phagocytic function mediated by both polymorphonuclears and monocytes was significantly impaired in GH-deficient subjects compared to controls (p < 0.003 for neutrophils, p < 0.007 for monocytes), while a significant increase of phagocytic activity was obtained during long-term rhGH replacement therapy (p < 0.02 for neutrophils, p < 0.001 for monocytes), thus suggesting that GH may affect the functional activity of circulating phagocyte cells. No significant differences were found in total granulocyte, monocyte and lymphocyte counts, T- and B-lymphocyte subsets and immunoglobulin levels, between GH-deficient patients and controls, and between values observed before and during rhGH substitution treatment.