As endogenous Cushing's syndrome (CS) in children occurs during a critical developmental period, when the majority of peak bone mass is acquired, we hypothesized that children with CS might be at an increased risk of osteoporosis. To determine the effects of CS on bone density, bone metabolism, and growth, we studied a 15-yr-old female identical twin pair, one of whom had CS (twin A), and the other of whom was healthy (twin B). Before therapy for CS, twin A showed a severe loss of bone mineral density [BMD; -3.2SD at the lumbar spine (LS)] compared to twin B (-0.1 SD), which in twin A was associated with low serum osteocalcin levels and urinary pyridinium cross-link excretion. Cure of CS in twin A led to a marked increase in these bone markers, suggesting a state of active bone remodeling. After 27 months of follow-up, even though twin A's BMD improved significantly, it still remained abnormal [-1.9 SD at LS compared with that of twin B (0 SD)], suggesting that twin A continued to be at increased long term risk of osteoporosis. In addition, as a consequence of CS, twin A's final height was 21 cm less than that of her identical twin. We recommend that all children with CS should have BMD monitored after treatment to determine the long term risk of osteoporosis.