In 22 acromegalic patients, microradiographic studies of biopsy samples from rib cortex Showed the proportion of forming surfaces was greater than the proportion of resorbing surfaces although both were increased (p < 0.01) and both correlated (p < 0.001) with serum concentration of immunoreactive growth hormone; in 4 patients, similar studies of predominantly trabecular bone from iliac crest or femoral neck showed that resorbing surfaces predominated. In 26 acromegalic patients, transmission scanning with an 125I source at midradius showed that bone diameter was increased (p < 0.05) and cortical bone mass was normal (17 patients) or increased (9 patients); in these same patients, an index based on trabecular pattern in the proximal femur showed values lower than normal (p < 0.001). The data indicate that acromegalics have increased bone width, increased cortical remodeling, and normal or increased cortical bone but decreased trabecular bone. These findings do not support the traditional belief that growth hormone excess in adults produces osteoporosis but are consistent with changes occurring as a consequence of reactivation of normal growth patterns with the restraint that radial bone growth and internal reorganization of trabecular bone can occur but linear growth cannot after epiphyseal plate closure.