OBJECTIVES: To investigate the effect of low-dose, long-term recombinant human growth hormone (rhGH) therapy on immune reconstitution in human immunodeficiency virus (HIV)-infected patients with focus on thymic index, density and output. DESIGN: Randomized, placebo-controlled, double-blind, single-centre trial. METHODS: Forty-six HIV-infected Caucasian men on highly active antiretroviral therapy, 21-60 years of age, were included. Twenty-eight patients were randomized to 0.7 mg/day rhGH and 18 patients to placebo, administrated as daily subcutaneous injections between 1300 and 1500 h for 40 weeks. Endpoints were changes from baseline in thymic size and thymic output measured as T-cell receptor rearrangement excision circles (TREC) frequency and total TREC content, and total and naive CD4 cells. RESULTS: Thymic density and thymic index increased in the GH group, compared with the placebo group (28 versus 4 Hounsfield units, P = 0.006 and 1 versus 0, P = 0.004). TREC frequency and total TREC content increased in the GH group, compared with the placebo group (37 versus -8%, P = 0.049 and 51 versus -14%, P = 0.026). Total CD4 cells and naive CD4+ cells increased insignificantly more in the GH than the placebo group [11.4%, 95% confidence interval (CI) -6.0 to 28.9; P = 0.19 and 18%, interquartile range (IQR) -4, 40 versus 13%, IQR -12, 39; P = 0.79]. Therapy was well tolerated. CONCLUSIONS: Daily treatment with a low dose rhGH of 0.7 mg for 40 weeks stimulated thymopoiesis expressed by thymic index, density and area, TREC frequency and total TREC content in CD4 cells in HIV-infected patients on highly active antiretroviral therapy.