Serum osteocalcin and alkaline phosphatase levels, as indexes of bone formation, and urinary calcium and hydroxyproline excretions relative to creatinine, as indexes of bone resorption, were measured in 10 obese women before and after two months of hypocaloric diet. In basal condition, serum osteocalcin, but not alkaline phosphatase levels, were higher in obese than in controls (7 +/- 0.4 vs 5.3 +/- 0.2 ng/ml). Urinary calcium/creatinine and hydroxyproline/creatinine ratios were also significantly higher than those in normals (0.37 +/- 0.05 vs 0.2 +/- 0.01 and 0.035 +/- 0.004 vs 0.02 +/- 0.002, respectively). After weight loss, serum osteocalcin significantly increased (9.5 +/- 0.5 ng/ml), while urinary calcium/creatinine and hydroxyproline/creatinine ratios fell to the normal values (0.23 +/- 0.03 and 0.026 +/- 0.001). In conclusion, it appears that obesity, at least in young women, is associated with a high bone turnover, which seems to be reversible with weight loss.