Transiliac bone biopsies were obtained from 36 growth hormone (GH) deficient men (mean age +/- SD, 28 +/- 4 years), of which 13 had an isolated GH deficiency and 23 had partial or complete hypopituitarism. The latter group was adequately substituted for the pituitary hormone deficiencies other than GH. Static histomorphometry was compared with eight controls, and dynamic histomorphometry was compared with six healthy men matched for age. Mean trabecular bone volume was not decreased and bone volume was high (> 30%) in ten patients. Osteoid thickness and mineralization lag time were slightly although not significantly higher than in controls. Osteoid surface, mineralizing surface and bone formation rate tended to be lower than in the controls. The eroded surface was significantly higher (p < 0.002) in the GH deficient patients. The results demonstrate that GH deficient patients do not show trabecular osteoporosis. The increased eroded surface together with normal to increased bone volume and bone surface suggests a prolonged reversal phase or a less sufficient coupling phenomenon.