CONTEXT Low bone mass is a characteristic feature of the adult GH deficiency (GHD) syndrome, but recent dual-energy x-ray absorptiometry (DXA) studies in patients with GH-receptor and GHRH-receptor gene mutations suggest that the situation is more complex. OBJECTIVE The objective was to define bone areal and volumetric densities and morphometry in hypopituitary adults. DESIGN The study was a cross-sectional case-controlled study performed between 1999 and 2001. SETTING The study was undertaken at an endocrine tertiary referral center. PATIENTS Thirty patients with GHD, 24 with GH insufficiency (GHI) [peak GH, 3-7 microg/liter (9-21 mU/liter)], and 30 age- and sex-matched controls were included for study. MAIN OUTCOME MEASURES DXA and peripheral quantitative computed tomography (pQCT) derived bone density and morphometry were measured. RESULTS No densitometric or morphometric abnormalities were detected in GHD patients who acquired their deficiency during adult life. GHD adults of childhood-onset (CO-GHD) showed decreased bone mineral density at the lumbar spine and hip on DXA. pQCT of the radius showed that CO-GHD patients have normal trabecular bone mineral density and only a 2% decrease in cortical density. Radial bone area was reduced 14.5%, cortical thickness 20%, and cortical cross-sectional area 23%, culminating in a reduction in cortical bone of 25%. The "apparent" low DXA bone density in CO-GHD adults therefore relates primarily to reduced cortical thickness and smaller bone area. DXA and pQCT data derived from adults with GHI revealed no evidence of densitometric or morphometric abnormalities. CONCLUSIONS 1) Adult-onset GHD patients have normal bone density and size. 2) CO-GHD adults have marginally reduced cortical density but significantly reduced cortical bone as a result of reduced cortical thickness and bone size. 3) GHI has no measurable impact on the skeleton.