Adults with growth hormone deficiency (GHD) exhibit low bone mineral density (BMD) which improves by growth hormone (GH) replacement therapy. The insulin-like growth factor (IGF) system has an established role in mediating the effects of GH on bone and IGF binding proteins (IGFBP)-4 and IGFBP-5 have been shown to modulate the effects of IGFs in bone. Therefore, we studied serum levels of IGFBP-4 and IGFBP-5 and their relationship to serum levels of bone biochemical markers and BMD in adults with GH deficiency (GHD) before and during GH therapy. Serum levels of IGFBP-5 and IGFBP-4 were measured on samples from 20 patients (11 males) 22–57 years of age. All had IGF-I serum values below –2 standard deviation score. The first 6 months were placebo controlled and all recieved 3 years of active treatment with the mean dose 0.23 ± 0.01 IU/kg/week divided into daily subcutaneous injections. Serum IGFBP-5 levels in GHD adults were low at baseline and positively related to total body, femoral neck, trochanter, and Ward's triangle BMD (r = 0.471, 0.549, 0.462, and 0.470, respectively, p < 0.05). The mean serum IGFBP-5 level increased by about 2-fold within 3 months after the initiation of GH therapy and was correlated with serum IGF-I (r = 0.719, 0.801, and 0.722 before and after 18 and 36 months, respectively, p < 0.001). A positive correlation between serum IGFBP-5 levels and lumbar spine BMD was found during GH treatment but not before. The percentage increase of serum IGFBP-5 after GH therapy showed a positive correlation with the percentage increase of total alkaline phosphate activity (r = 0.347 p < 0.05). In contrast to IGFBP-5, serum IGFBP-4 levels were positively related to body mass index (r = 0.607, p < 0.01). Baseline serum IGFBP-4 levels also correlated with total body, femoral neck, trochanter, and Ward's triangle BMD (r = 0.502, 0.590, 0.612, and 0.471, respectively, p < 0.05). The mean serum IGFBP-4 level was increased by 25% within 3 months after initiation of GH therapy and did not correlate with serum IGF-I levels. Although the above findings are consistent with the idea that GH-induced changes in serum IGFBP-5 and IGFBP-4 levels may in part mediate the anabolic effects of GH on bone tissue in adults with GHD, further studies are needed to establish the cause and effect relationship.