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Bone mineral density (BMD) is influenced by growth factors, such as growth hormone (GH) and insulin-like growth factor-I (IGF-I). The in vivo bioassay for GH (bioGH) provides a more physiologically relevant measurement than an in vitro immunoassay, since bioGH is quantified on a biological outcome.To determine if bioGH and components of the IGF-I system were associated with BMD in age-matched men (M; n=41, 19.1+/-0.2 year, 70+/-3 kg, 163+/-25 cm) and women (W; n=39, 18.6+/-0.3 year, 66+/-3 kg, 141+/-15 cm).Blood was analyzed for growth-related hormones [bioGH, immunoreactive growth hormone (iGH), IGF-I and associated binding proteins], and BMD was measured by pDXA, pQCT, and central DXA (spine, hip). For the bioGH assay, hypophysectomizied female Sprague-Dawley rats were injected with a s.c. bolus of either a GH standard or unknown (each subject's plasma) in four daily injections. The tibia was then examined for epiphyseal growth plate width from which bioGH concentrations were extrapolated.M had greater (P0.05) calcaneal BMD when measured by pDXA (M: 1.27+/-0.02; W: 1.14+/-0.02 g/cm2), while pQCT-assessed BMD at the tibia was not different (M: 777+/-16; W: 799+/-16 g/cm2). bioGH was similar between M (5388+/-800 microg/L) and W (4282+/-643 microg/L) and was not correlated with BMD. The only BMD-related biomarkers in women were acid-labile subunit (ALS; r=0.40) and IGFBP-3 (r=0.42) with DXA-measured spine and femoral neck BMD, and ALS (r=0.47) with pQCT-assessed tibial BMD and cortical thickness, respectively.Although bioGH was not associated with BMD, IGF-I and associated binding proteins (IGFBP-3 and ALS) emerged as correlates in W only.